These French Onion Roast Beef Sandwiches are the ultimate comfort food—loaded with caramelized onions, melted Gruyere, and tender rare roast beef on a toasted baguette.

Served with a rich homemade au jus for dipping, every bite is juicy, cheesy, and crave-worthy.

Why You’ll Love This Recipe

Melty, cheesy goodness with Gruyere

Savory au jus made from scratch

Toasty baguette with tender rare roast beef

Perfect for game day or cozy nights

Ingredient Highlights

Sandwich Base

French baguette: Cut into 4 sandwich rolls

Rare roast beef: Sliced thin for tenderness

Onions & Flavor

Yellow onions: Caramelized to golden perfection

Butter & fresh thyme: Adds depth to the onions

Salt, black pepper, red pepper flakes: For seasoning

Au Jus

Cooking sherry: Enhances flavor and deglazes the pan

Beef stock: Rich and savory base

Cornstarch + water: For a lightly thickened dipping broth

Cheese

Gruyere: Nutty, melty, and perfect for French onion sandwiches

Pro Tips Before You Start

Cook the onions low and slow—don’t rush them.

Use high-quality beef stock for deeper flavor.

Shred your own Gruyere for better melting.

Don’t skip the foil wrap—it melts the cheese perfectly.

How to Make French Onion Roast Beef Sandwiches

Step 1: Prep the Baguette

Slice the baguette into 4 equal parts.

Cut each section horizontally, stopping about 3/4 of the way through to create a sandwich pocket.

Step 2: Preheat the Oven

Set your oven to 500°F so it’s ready to toast the sandwiches.

Step 3: Fill the Sandwiches

Evenly divide the roast beef among the 4 baguette sections.

Place each sandwich on a piece of aluminum foil and set aside.

Step 4: Caramelize the Onions

In a large skillet over medium heat, melt the butter.

Add thyme and sliced onions.

Stir well and cook for 20–25 minutes, stirring occasionally, until onions are soft and golden.

Step 5: Season and Deglaze

Add salt, black pepper, and red pepper flakes.

Stir and cook for 2 minutes.

Add the sherry and cook for 3–4 more minutes until the onions absorb the liquid.

Step 6: Make the Au Jus

Pour in the beef stock and bring to a boil over medium-high heat.

Reduce to a simmer and cook for 5 minutes.

Step 7: Thicken the Sauce

Mix cornstarch with water to form a slurry.

Stir into the simmering sauce and cook for 1–2 minutes to slightly thicken.

Step 8: Prep the Onions and Au Jus

Using a slotted spoon, transfer the onions to a bowl.

Divide the au jus into ramekins for serving.

Step 9: Assemble the Sandwiches

Top each sandwich with shredded Gruyere, followed by a generous scoop of the cooked onions.

Step 10: Toast the Sandwiches

Wrap each sandwich tightly in foil, place on a baking sheet, and toast in the oven for 5 minutes until cheese is melted.

Step 11: Serve and Dip

Carefully unwrap each sandwich and serve immediately with a ramekin of warm au jus for dipping.

What to Serve It With

Crisp pickles or a dill cucumber salad

French fries or roasted potatoes

Light coleslaw or green salad

Variations & Substitutions

Swap Gruyere with provolone or Swiss

Use turkey or pork roast instead of beef

Add horseradish aioli or Dijon mustard for a spicy kick

Storage & Leftovers

Refrigerate leftovers in foil for up to 3 days.

Reheat wrapped in foil at 350°F until warmed through.

Freeze caramelized onions and au jus separately if prepping ahead.

FAQs

Can I make the onions ahead of time?
Yes! Caramelized onions can be stored in the fridge for 3–4 days or frozen for later.

Can I use a different bread?
Sure! Ciabatta or hoagie rolls work well too.

What can I use instead of cooking sherry?
Try a splash of dry white wine or skip it entirely and add a little extra beef stock.

Is this recipe spicy?
Only mildly, from red pepper flakes—you can leave them out.

Can I melt the cheese another way?
Yes, you can also use a broiler instead of the foil method.

Do I need to thicken the au jus?
It’s optional! The slurry gives it body but can be skipped for a thinner dip.

What cheese is closest to Gruyere?
Swiss or Emmental are great substitutes if Gruyere isn’t available.

Final Thoughts

If you’re craving something rich, warm, and packed with umami, these French Onion Roast Beef Sandwiches are exactly what you need.

They bring together the classic flavors of French onion soup and a cheesy melt in one irresistible bite.

With homemade au jus and bubbling Gruyère, they’re comfort food done right.