You know that moment when you take a bite of something and just pause? Like… you actually stop chewing because your brain needs a second to process how good it is?
That’s exactly what happened the first time I made French Onion Smashburgers with Gruyère in my own kitchen.
I love a classic smashburger. I also love French onion soup—the deep, caramelized onions, the nutty melted cheese, that savory richness. One night I stared at a skillet full of onions and thought, “Why am I not putting this on a burger?” And honestly? Best decision ever.
Today I’m sharing exactly how I make them, including the small tweaks that make a huge difference. If you want a juicy smashburger with crispy edges, sweet caramelized onions, and perfectly melted Gruyère cheese, you’re in the right place.
Why This French Onion Smashburger Actually Works
Let’s break it down.
A good smashburger relies on three things:
- High heat
- Proper smashing technique
- Fat content in the beef
When you add slow-cooked onions and Gruyère, you introduce sweetness, depth, and that slightly nutty, salty melt that takes everything up a level.
Ever wondered why French onion soup tastes so luxurious even though it’s mostly onions? It’s because you cook the onions low and slow until they transform. We use that same trick here.
This recipe naturally hits high-value search phrases like:
- French onion smashburger
- smashburger recipe
- caramelized onion burger
- Gruyère cheeseburger
- homemade smash burgers
And yes, it tastes even better than it sounds.
Ingredients You’ll Need
For the Caramelized Onions
- 3 large yellow onions, thinly sliced
- 2 tablespoons unsalted butter
- 1 tablespoon olive oil
- ½ teaspoon salt
- 1 teaspoon sugar (optional, but helps browning)
- 2 tablespoons beef broth
For the Smash Burgers
- 1 ½ pounds 80/20 ground beef
- Salt and freshly cracked black pepper
- 4 brioche buns
- 4 slices Gruyère cheese
- 1 tablespoon Dijon mustard
- Optional: fresh thyme
The Equipment That Makes a Big Difference
You can absolutely make this in a regular skillet. But if you want restaurant-level crust, I strongly recommend:
- A heavy cast iron skillet
- A sturdy metal spatula
- Parchment paper squares
I personally use the Lodge 10.25 Inch Cast Iron Skillet . It retains heat beautifully and gives that crispy smashburger crust every single time.
For smashing, I use the Cuisinart Stainless Steel Spatula . Thin edge, strong handle. No bending, no frustration.
And when it comes to cheese? I grate Emmi Le Gruyère AOP Swiss Cheese . It melts like a dream and gives that authentic French onion flavor.
How to Make Perfect Caramelized Onions
Step 1: Start Low and Slow
Heat butter and olive oil in your skillet over medium-low heat. Add the onions and salt. Stir them well so they coat evenly.
Now here’s where patience comes in.
You cook these for 30–40 minutes, stirring occasionally. They soften first. Then they turn translucent. Then they deepen into golden brown.
If you rush this step, you won’t get that rich sweetness. Trust me.
Step 2: Add Depth
Once the onions caramelize, add beef broth. Scrape the bottom of the pan and let it cook down.
This small step adds serious flavor. I sometimes toss in a pinch of fresh thyme for extra aroma.
Set the onions aside.
How to Make Smash Burgers the Right Way
Step 1: Form Loose Balls
Divide ground beef into 4 loosely packed balls. Don’t overwork the meat. Ever pressed beef too tightly and ended up with dense burgers? Yeah. We don’t do that here.
Step 2: Heat the Skillet Until Smoking
Crank your cast iron skillet to high heat. It should be hot enough that a drop of water sizzles instantly.
This creates that crispy crust smashburgers are famous for.
Step 3: Smash and Season
Place one beef ball onto the skillet.
Immediately place parchment paper on top and press down firmly with your spatula. Hold for 10 seconds.
Season with salt and pepper.
That smashing action forces maximum surface contact, which creates those lacy, crispy edges.
Step 4: Flip Once
Cook 2–3 minutes until deeply browned.
Flip. Add Gruyère immediately. Let it melt for about 1–2 minutes.
No flipping back and forth. Just once.
Toasting the Brioche Buns
Never skip this.
Lightly butter the inside of your brioche buns and toast them in the skillet until golden brown.
Toasted buns prevent sogginess and add texture contrast.
IMO, brioche works best because it balances the savory richness with slight sweetness 🙂
Assembly: The Magic Moment
Spread a thin layer of Dijon mustard on the bottom bun.
Add:
- Smashburger with melted Gruyère
- Generous scoop of caramelized onions
- Optional fresh thyme
Top with bun.
Press slightly.
Take a bite.
Pause.
Smile.
Tips That Make This Better Than Most Recipes Online
After testing this more times than I can count, here’s what actually matters:
1. Use 80/20 Beef
Lean beef dries out. Fat equals flavor.
2. Don’t Pre-Season the Meat
Salt draws out moisture. Season only after smashing.
3. Shred Your Own Gruyère
Pre-shredded cheese doesn’t melt the same way.
4. Let the Onions Go Darker Than You Think
That deep brown color equals maximum sweetness.
Flavor Variations (If You Want to Play Around)
- Add a splash of balsamic to onions
- Add crispy shallots for crunch
- Melt two thinner slices of cheese instead of one thick slice
- Add a splash of Worcestershire to beef
Ever tried adding a tiny bit of black pepper directly into caramelized onions? It hits differently.
Why This Recipe Feels Restaurant-Level
Most top burger recipes focus on technique. Most French onion recipes focus on onions.
This combines both worlds.
You get:
- Crispy-edged smashburger
- Sweet slow-cooked onions
- Nutty melted Gruyère
- Buttery brioche
It tastes like something you’d pay $18 for at a gastropub.
Except you made it in your own kitchen.
Make Ahead & Storage Tips
Caramelized onions store well for 3–4 days in the fridge.
Reheat gently before assembling burgers.
You can even freeze onions for up to 2 months.
Burgers taste best fresh, but you can reheat patties in a skillet over medium heat.
Nutrition Overview (Per Serving Approximate)
- Calories: 620
- Protein: 35g
- Fat: 38g
- Carbs: 32g
Remember, this is comfort food. We aren’t counting lettuce leaves here 😉
Ingredients
Method
- Heat butter and olive oil in skillet over medium-low heat.
- Add onions and salt. Cook 30–40 minutes, stirring occasionally.
- Add beef broth and cook until absorbed. Set aside.
- Preheat cast iron skillet to high heat.
- Form beef into 4 loose balls.
- Place ball in skillet and smash firmly with spatula using parchment paper.
- Season with salt and pepper.
- Cook 2–3 minutes until browned. Flip once.
- Add Gruyère and cook 1–2 minutes until melted.
- Toast brioche buns.
- Assemble with Dijon, burger, onions, and top bun.
Notes
- Use high heat for proper crust formation.
- Do not overwork ground beef.
- Caramelize onions slowly for best flavor.
- Toast buns to prevent sogginess.