Quick Recipe Version (TL;DR)
Quick Ingredients
- 1 tbsp olive oil
- 1 large yellow onion, thinly sliced
- Pinch kosher salt, 1 tsp balsamic (optional)
- 1 crusty baguette (16–18 in / ~280 g)
- 4 oz sharp cheddar, thinly sliced
- 12 oz thin-sliced roast beef
- 2 cups baby arugula
- Horseradish cream: 1/3 cup mayo, 3 tbsp prepared horseradish, 1 tsp Dijon, 1 tsp lemon juice, black pepper, pinch salt
Do This
- 1. Caramelize onion in 1 tbsp olive oil with a pinch of salt over medium-low, 18–22 minutes; splash in 1 tsp water or balsamic at the end.
- 2. Stir together mayo, horseradish, Dijon, lemon, pepper, and a pinch of salt.
- 3. Heat oven to 375°F (190°C). Split baguette lengthwise (leave a hinge) and toast cut-side up 6–8 minutes.
- 4. Top bottom half with cheddar; return to oven 2–3 minutes to melt (or broil 1–2 minutes).
- 5. Optional: Warm roast beef in a skillet with 1 tbsp water, 60–90 seconds.
- 6. Layer beef, caramelized onions, arugula, and generous horseradish cream. Close, slice into 4, and serve.
Why You’ll Love This Recipe
- Balanced bite: peppery arugula, sharp cheddar, sweet onions, and zippy horseradish.
- Restaurant-quality sandwich you can make in under 40 minutes.
- Easy to scale for game day or a casual lunch.
- Great make-ahead components: onions and sauce keep well for days.
Grocery List
- Produce: 1 large yellow onion, 1 lemon, 2 cups baby arugula
- Dairy: 4 oz sharp cheddar (white or yellow)
- Pantry: Baguette, olive oil, kosher salt, black pepper, balsamic vinegar (optional), mayonnaise, Dijon mustard, prepared horseradish, deli roast beef
Full Ingredients
Caramelized Onions
- 1 tbsp olive oil
- 1 large yellow onion, halved and thinly sliced (about 3 cups / 280 g)
- 1/4 tsp kosher salt
- 1/2 tsp granulated sugar (optional, helps browning)
- 1 tsp balsamic vinegar or water, for deglazing (optional)
Creamy Horseradish Sauce
- 1/3 cup mayonnaise (75 g)
- 3 tbsp prepared horseradish, drained if watery (about 45 g)
- 1 tsp Dijon mustard
- 1 tsp fresh lemon juice
- 1/4 tsp freshly ground black pepper
- Pinch kosher salt
Sandwich Assembly
- 1 crusty French baguette (16–18 in / ~280 g), split lengthwise and hinged
- 4 oz sharp cheddar, thinly sliced (113 g)
- 12 oz thin-sliced roast beef (340 g)
- 2 cups baby arugula (about 2 oz / 60 g)
- Freshly ground black pepper, to finish
- Flaky sea salt, a pinch (optional)
Step-by-Step Instructions
Step 1: Caramelize the onions
Heat 1 tbsp olive oil in a wide skillet over medium-low heat. Add the sliced onion, 1/4 tsp kosher salt, and optional 1/2 tsp sugar. Cook, stirring every few minutes, until the onions are deep golden and jammy, 18–22 minutes. If the pan looks dry or browned bits form, deglaze with 1–2 tsp water. Stir in 1 tsp balsamic at the end if you like a touch of tang. Set aside.
Step 2: Make the horseradish cream
In a small bowl, mix mayonnaise, prepared horseradish, Dijon, lemon juice, black pepper, and a pinch of salt. Taste and add more horseradish for extra heat if desired. Refrigerate until needed.
Step 3: Toast the baguette
Heat the oven to 375°F (190°C). Split the baguette lengthwise, keeping a hinge so it opens like a book. Place cut sides up on a baking sheet and toast until the edges are crisp but not dry, 6–8 minutes.
Step 4: Melt the cheddar
Lay the cheddar slices evenly over the bottom half of the warm baguette. Return to the oven until the cheese is just melted, 2–3 minutes. For faster browning, switch to the broiler on high and melt for 1–2 minutes, watching closely to avoid burning.
Step 5: Warm the roast beef (optional) and build
For a deli-style warm sandwich, pile the roast beef into the still-warm skillet with 1 tbsp water. Warm over medium heat, tossing just until heated through and steamy, 60–90 seconds; do not overcook. Layer the warm beef onto the melted cheddar, then spoon on the caramelized onions.
Spread a generous layer (2–3 tbsp) of horseradish cream on the cut side of the top half of the baguette. Add the arugula over the onions. Season with a few grinds of black pepper and a pinch of flaky salt if using.
Step 6: Close, slice, and serve
Close the sandwich and press gently to help it hold together. Use a serrated knife to cut into 4 equal portions. Serve immediately with extra horseradish cream on the side.
Pro Tips
- Drain watery horseradish before mixing so the sauce stays thick and spreadable.
- Keep a hinge on the baguette so fillings stay put and slicing is easier.
- Warm roast beef quickly with a splash of water to keep it juicy without overcooking.
- Let the onions go low and slow—if they brown too fast, lower the heat and add a teaspoon of water.
- Build cheese on the bottom and greens on top to prevent soggy bread.
Variations
- Blue & watercress: Swap cheddar for crumbled blue cheese and use watercress instead of arugula.
- Pickled kick: Replace caramelized onions with quick-pickled red onions and add sliced hot cherry peppers.
- Garlic butter melt: Brush the cut sides of the baguette with garlic butter before toasting for extra richness.
Storage & Make-Ahead
Caramelized onions and horseradish cream can be made up to 4 days ahead; refrigerate in airtight containers. For best texture, assemble the sandwich just before serving. If packing for lunch, keep arugula and sauce separate; assemble and add sauce just before eating. Leftover assembled sandwiches can be wrapped and refrigerated for up to 24 hours, though the bread will soften. Re-crisp under a hot broiler for 1–2 minutes, then add fresh arugula.
Nutrition (per serving)
Approx. 640 calories; 38 g protein; 33 g fat; 44 g carbohydrates; 3 g fiber; 1150 mg sodium. Values are estimates and will vary with brands and portion sizes.
