Quick Recipe Version (TL;DR)
Quick Ingredients
- 1 small red onion, thinly sliced
- 1/2 cup distilled white vinegar
- 1/2 cup water (hot)
- 1 tbsp granulated sugar
- 1 1/2 tsp kosher salt, divided
- 1/3 cup mayonnaise
- 2 tbsp Dijon mustard
- 1 1/2 tbsp prepared horseradish (drained)
- 1 tsp fresh lemon juice
- 1/2 tsp garlic powder
- 8 oz sirloin steak (about 3/4-inch thick)
- 1/2 tsp black pepper
- 1 tbsp neutral oil (canola or avocado)
- 2 hoagie rolls (about 6–7 inches each), split
- 6 oz sharp white cheddar, sliced
- 2 tsp softened butter (for grilling), or 2 tsp mayonnaise
Do This
- 1. Quick-pickle onions: pour hot vinegar brine over sliced onions; rest 15 minutes.
- 2. Stir together mayo, Dijon, horseradish, lemon, garlic powder, and a pinch of salt.
- 3. Season steak with 1 tsp kosher salt and pepper; sear in oil over medium-high heat 3–4 minutes per side.
- 4. Rest 5 minutes, slice very thin across the grain, then warm slices 30–60 seconds in the pan.
- 5. Build sandwiches: spread sauce on cut sides, add cheddar and warm steak, top with drained pickled onions.
- 6. Butter outsides; press in a panini press at 375°F for 3–5 minutes (or skillet 3–4 minutes per side).
- 7. Rest 1 minute, slice, and serve hot and melty.
Why You’ll Love This Recipe
- It captures the bold, tangy flavor combo: savory steak, sharp white cheddar, and zippy horseradish–mustard spread.
- Quick pickled red onions add crunch and brightness in just 15 minutes.
- Pressing the sandwich gives you that crisp, toasty exterior with a gooey, melty center.
- Great for weeknights, and the components can be prepped ahead for fast assembly.
Grocery List
- Produce: 1 small red onion, lemon
- Dairy: sharp white cheddar (about 6 oz), butter (optional for grilling)
- Meat: sirloin steak (8 oz)
- Bakery: 2 hoagie rolls (6–7 inches each)
- Pantry: distilled white vinegar, granulated sugar, kosher salt, black pepper, mayonnaise, Dijon mustard, prepared horseradish, garlic powder, neutral oil (canola or avocado)
Full Ingredients
Quick Pickled Red Onions (makes about 1 cup)
- 1 small red onion (about 6–7 oz), halved and thinly sliced
- 1/2 cup distilled white vinegar
- 1/2 cup hot water (not boiling; about 140–160°F)
- 1 tbsp granulated sugar
- 1 tsp kosher salt
Horseradish–Mustard Spread
- 1/3 cup mayonnaise
- 2 tbsp Dijon mustard
- 1 1/2 tbsp prepared horseradish, drained (adjust to taste)
- 1 tsp fresh lemon juice
- 1/2 tsp garlic powder
- 1/2 tsp kosher salt
Steak
- 8 oz sirloin steak (about 3/4-inch thick)
- 1 tsp kosher salt
- 1/2 tsp black pepper
- 1 tbsp neutral oil (canola or avocado)
Sandwich Assembly
- 2 hoagie rolls (6–7 inches each), split lengthwise
- 6 oz sharp white cheddar, thinly sliced
- 2 tsp softened butter (for the outside of the bread), or 2 tsp mayonnaise
Step-by-Step Instructions
Step 1: Quick-pickle the red onions
In a heat-safe bowl or pint jar, add the thinly sliced red onion.
In a measuring cup, whisk together 1/2 cup distilled white vinegar, 1/2 cup hot water (140–160°F), 1 tbsp sugar, and 1 tsp kosher salt until dissolved. Pour the mixture over the onions and press them down so they’re mostly submerged.
Let sit at room temperature for 15 minutes. (You can leave them longer; they’ll just get more tender and punchy.) Drain before using so the panini doesn’t get soggy.
Step 2: Make the horseradish–mustard spread
In a small bowl, stir together 1/3 cup mayonnaise, 2 tbsp Dijon mustard, 1 1/2 tbsp prepared horseradish (drained), 1 tsp lemon juice, 1/2 tsp garlic powder, and 1/2 tsp kosher salt.
Taste and adjust: add a little more horseradish for extra heat, or an extra 1/2 tsp lemon juice for more brightness.
Step 3: Season the steak
Pat the steak dry with paper towels. Season both sides evenly with 1 tsp kosher salt and 1/2 tsp black pepper.
If you have time, let it sit for 5 minutes at room temperature while you heat the pan. (This helps it cook more evenly.)
Step 4: Sear the steak and rest
Heat a cast-iron skillet or heavy pan over medium-high heat for 2 minutes. Add 1 tbsp neutral oil and swirl to coat.
Sear the steak for 3–4 minutes on the first side, then flip and cook 3–4 minutes on the second side for medium-rare to medium (aim for 130–140°F internal temperature). If you prefer it more done, cook to 150°F for medium-well.
Transfer steak to a cutting board and rest for 5 minutes so the juices redistribute.
Step 5: Slice thin and warm the steak for sandwich-perfect texture
Slice the steak very thinly across the grain. Thin slices are key for that deli-style, tender bite.
For a Panera-style warm filling, return the skillet to medium heat. Add the sliced steak and toss for 30–60 seconds, just until warmed through. (You’re not re-cooking it, just heating it so the cheese melts nicely.)
Step 6: Assemble the paninis
Open the hoagie rolls. Spread the horseradish–mustard sauce on the cut sides (about 1 1/2 tbsp per roll, or to taste).
Layer sharp white cheddar on the bottom halves, then pile on the warm sliced steak. Top with a generous pinch of drained pickled red onions, then close the sandwiches.
Step 7: Press until crisp and melty
Panini press method (best): Preheat your panini press to 375°F. Lightly butter the outside of each sandwich with about 1 tsp butter per sandwich (or spread a thin layer of mayo). Press for 3–5 minutes until deeply golden and the cheddar is fully melted.
Skillet method: Heat a skillet or grill pan over medium heat. Butter the outside of the bread. Cook 3–4 minutes per side, pressing firmly with a second heavy skillet or a foil-wrapped brick, until crisp and the cheese is melted. Keep heat at medium so the bread browns without burning before the cheese melts.
Step 8: Rest, slice, and serve
Let the paninis rest for 1 minute (this helps the cheese set slightly so it doesn’t all slide out).
Slice on the bias and serve immediately. If you like, add a small extra pinch of drained pickled onions right before serving for extra crunch.
Pro Tips
- For ultra-thin steak slices: Chill the cooked steak in the freezer for 10 minutes before slicing. It firms up and slices cleaner.
- Drain the onions well: Too much brine can make the bread soggy and can steam the crust instead of crisping it.
- Cheese placement matters: Put cheddar directly against the bread on at least one side so it melts into a “glue” that holds the steak and onions in place.
- Keep the heat moderate when pressing: Medium heat gives the cheese time to melt before the bread gets too dark.
- No panini press? Use a skillet plus a second heavy pan to press. Parchment paper between the sandwich and the weight keeps things tidy.
Variations
- Roast beef shortcut: Replace the cooked steak with 10–12 oz thinly sliced deli roast beef. Warm it in a skillet with 2 tbsp water for 45 seconds before assembling.
- Caramelized onion swap: Skip pickling and use caramelized onions for a sweeter, richer sandwich (cook sliced onions in 1 tbsp butter over medium-low for 25–30 minutes).
- Spicier spread: Add 1/2 tsp hot sauce or 1/4 tsp cayenne to the horseradish–mustard spread.
Storage & Make-Ahead
Make-ahead: Pickled onions can be made up to 2 weeks ahead and stored covered in the refrigerator. The horseradish–mustard spread keeps for 5 days refrigerated in an airtight container. Cooked steak keeps for 3–4 days refrigerated.
Best practice: Store components separately, then assemble and press right before eating for the crispiest bread.
Reheating: Reheat an already-pressed panini in a 350°F oven for 8–10 minutes (flip halfway), or in a dry skillet over medium-low heat for 3–4 minutes per side. Avoid the microwave if you want to keep the crust crisp.
Nutrition (per serving)
Approximate, for 1 of 2 paninis: 780 calories, 45 g protein, 58 g carbohydrates, 38 g fat, 4 g fiber, 7 g sugar, 1450 mg sodium.
