In a large skillet, heat the butter and olive oil over medium heat.
Add the sliced onions and cook, stirring occasionally, until they begin to soften, about 10 minutes.
Sprinkle in the sugar and continue cooking, stirring frequently, until the onions are golden brown and caramelized (this will take around 20-25 minutes).
If desired, add the balsamic vinegar in the last 5 minutes of cooking for added depth of flavor.
Season with salt and pepper to taste, then set aside.
2. Prepare the Burger Patties:
While the onions are caramelizing, form the ground beef into 4 equal-sized patties. Press a small indent in the center of each patty to help them cook evenly.
Season the patties with salt, pepper, and Worcestershire sauce (if using).
Heat a grill or skillet over medium-high heat and cook the patties for about 4-5 minutes per side, or until they reach your desired level of doneness. Add the cheese slices to the patties during the last minute of cooking, covering with a lid to melt.
3. Make the French Onion Soup Sauce:
In a separate saucepan, melt the butter over medium heat. Add the flour and stir to create a roux. Cook for 1-2 minutes, until the flour is lightly golden.
Slowly whisk in the beef broth and white wine (if using), making sure there are no lumps.
Stir in the thyme, and cook until the sauce thickens slightly, about 3-4 minutes. Taste and adjust with salt and pepper as needed.
4. Toast the Buns:
While the burgers and sauce are cooking, lightly toast the burger buns on a grill or in a skillet until golden brown.
5. Assemble the Burgers:
Spread a spoonful of the French onion soup sauce on the bottom half of each toasted bun.
Place the cooked burger patties on top of the sauce.
Pile the caramelized onions on top of each patty, followed by a slice of melted Gruyère cheese.
Optionally, garnish with fresh thyme for extra flavor and a beautiful presentation.
6. Serve:
Top with the other half of the bun and serve your French Onion Soup Burgers hot. They pair wonderfully with crispy fries or a light salad.
