Instructions

• Heat 1 tablespoon extra-virgin olive oil and 1 tablespoon butter in a large nonstick or cast-iron skillet over medium to medium-low heat until the butter melts. Add 2 medium sweet onions (halved and thinly sliced) and cook, stirring frequently, until golden and caramelized, about 25 minutes.
• Stir in 1 teaspoon balsamic vinegar and 1/4 teaspoon dried thyme, remove the skillet from the heat, and set the onions aside.
• In a small bowl, combine 1/4 cup mayonnaise, 2 tablespoons Dijon mustard, and 1 minced garlic clove. Cover and refrigerate this dijonnaise until assembly.
• In a medium bowl, gently mix 1 1/2 pounds ground chuck with 1 packet dry French onion soup mix until just combined. Divide into 4 equal portions and shape into 4 patties.
• Heat a cast-iron skillet over medium-high heat until hot. Add the patties and cook without moving for 4 minutes.
• Flip the patties and cook an additional 3 minutes. Sprinkle 1 cup shredded Gruyère cheese evenly over the patties, reduce the heat to low, cover the skillet, and cook 1 minute to melt the cheese.
• Toast the 4 brioche buns (cut side down in a skillet or in a toaster) until golden.
• Assemble each burger: place some of the baby arugula on the bottom bun, spread a portion of the dijonnaise on top of the arugula, add a cheeseburger patty, top with caramelized onions, and finish with the bun top. Serve immediately.