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.
