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This decadent, incredibly juicy burger recipe can be easily whipped up on a weeknight, but is impressive enough for company.
3 tbsp unsalted butter, divided
1 Spanish onion, sliced into rounds
1 cup beef broth
500 g lean ground beef
1 tbsp finely chopped thyme
2 tsp Dijon mustard
4 slices Gruyère cheese
4 brioche burger buns, split
1/2 cup mayonnaise
1 tbsp chopped parlsey
1 cup arugula, (optional)
Melt butter in a large frying pan over medium. Reserve 1 tbsp in a small bowl. Add onion to pan and cook, stirring often, until softened, about 8 min. Pour in broth and continue cooking, stirring occasionally, until onion is very soft, about 15 min.
Meanwhile, combine beef with thyme and Dijon in a large bowl. Divide into 4 portions, then form into four 4-in. patties. Sprinkle both sides with salt and ¼ tsp pepper.
Preheat barbecue or grill pan on medium. Oil grill. Cook patties, lid closed, until cooked through, 5 to 6 min per side. Melt a slice of cheese on each patty, lid closed, 2 min.
Brush reserved 1 tbsp butter on insides of brioche buns, then place on grill to toast, about 2 min.
Meanwhile, stir mayo with parsley and remaining 3/4 tsp pepper in a small bowl.
Spread bottom buns with pepper mayo. Layer with patties, sautéed onions and arugula. Sandwich with top bun.