French Onion Roast Beef Sandwiches with Au Jus

These French Onion Roast Beef Sandwiches are rich, savory, and dripping with cheesy, oniony goodness.

Stuffed with layers of rare roast beef, caramelized onions, melted Gruyère, and served with a warm au jus for dipping, they’re the ultimate comfort food for cozy nights or weekend gatherings.

Why You’ll Love This Recipe

Layers of bold, beefy flavor in every bite

Melty Gruyère cheese with caramelized onions = irresistible

Homemade au jus makes it feel gourmet

Perfect for feeding a crowd or a hearty weeknight dinner

Easily prepped ahead and reheated

Ingredient Highlights

Rare roast beef: Thinly sliced for easy stacking and quick warming.

Yellow onions: Slowly caramelized with thyme for deep, sweet flavor.

Gruyère cheese: Aged and nutty, it melts beautifully over the beef.

French baguette: Crisp on the outside and soft inside for holding all the juices.

Cooking sherry + beef stock: For a rich, flavorful au jus perfect for dipping.

Pro Tips Before You Start

Use a sharp knife or mandoline to thinly slice the onions evenly.

Keep the heat low and stir often when caramelizing onions—it’s worth the time.

Wrap sandwiches tightly in foil before baking to keep everything hot and melty.

Don’t skip the au jus—it takes these sandwiches over the top!

If you’re short on time, use store-bought French onion soup as a shortcut for au jus.

How to Make French Onion Roast Beef Sandwiches

 Step 1: Prep the Bread

Slice the baguette into 4 equal pieces.

Then slice each piece horizontally, leaving one edge intact so they open like a book.

Step 2: Preheat the Oven

Set your oven to 500°F.

Step 3: Stuff the Sandwiches

Evenly divide the rare roast beef among the 4 bread sections.

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

Step 4: Caramelize the Onions

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

Add thyme and onions.

Stir frequently and cook for 20-25 minutes until golden and soft.

Step 5: Make the Au Jus

Season the onions with salt, black pepper, and red pepper flakes.

Stir in the sherry and cook for 3-4 minutes until absorbed.

Add beef stock and bring to a boil. Lower heat and simmer for 5 minutes.

Step 6: Thicken the Broth

In a small bowl, mix cornstarch with water to make a slurry.

Add it to the simmering broth and stir for another minute or two until slightly thickened.

Step 7: Assemble the Sandwiches

Use a slotted spoon to remove onions from the skillet and place them in a bowl.

Pour the au jus into ramekins for serving.

Top each sandwich with shredded Gruyère and caramelized onions.

Step 8: Bake

Wrap each sandwich in foil and place on a baking sheet.

Bake for 5 minutes until the cheese is melted and bread is warm.

Step 9: Serve

Remove from the oven, unwrap, and serve hot with au jus on the side for dipping.

What to Serve It With

Classic French fries or oven-roasted potatoes

Pickles or a crisp green salad

Coleslaw or roasted Brussels sprouts

A bold red wine or cold lager

Variations & Substitutions

Swap Gruyère with Swiss, provolone, or mozzarella

Add horseradish mayo or mustard for extra zing

Use a crusty ciabatta roll instead of baguette

Try caramelizing shallots instead of onions for a sweeter twist

Make it vegetarian by using mushrooms and veggie broth

Storage & Leftovers

Store cooled sandwiches (without au jus) in foil or an airtight container in the fridge for up to 3 days

Reheat wrapped in foil at 350°F for 10-15 minutes

Store au jus separately in the fridge for up to 4 days

Freeze onions and au jus separately for up to 1 month

FAQs

 Can I make these ahead of time?
Yes! Assemble the sandwiches without baking and store them in the fridge.

Bake just before serving.

Can I use deli roast beef?
Absolutely. Just make sure it’s sliced thin and not too salty.

What is cooking sherry, and can I substitute it?
Cooking sherry adds depth.

You can sub with dry white wine or omit it entirely.

Do I need to make the au jus from scratch?
You can use store-bought au jus or French onion soup in a pinch, but homemade tastes best.

How do I keep the bread from getting soggy?
Don’t overfill with sauce and bake sandwiches wrapped in foil to lock in heat but keep bread intact.

What other cheeses work well?
Provolone, fontina, or smoked gouda all melt well and taste amazing with roast beef.

Can I double the recipe?
Yes, it scales beautifully.

Just use a second baking sheet and rotate during oven time.

Final Thoughts

These French Onion Roast Beef Sandwiches are my go-to when I want to impress without fuss.

The rich au jus, sweet onions, and melty cheese layered in warm baguettes are simply unbeatable. Dip, bite, and enjoy every flavorful moment.