Burger with fried onions

Ingredients:

• 700 g of ground beef
• 1 tbsp vegetable oil + more as needed
• 2 onions, very thinly sliced ​​(preferably with a mandoline)
• 8 slices American cheese
• 4 hamburger buns
• Ketchup, mustard and/or pickle slices, for serving

Preparation:

1. Preheat oven to 120°C.
2. In a large bowl, combine the ground beef with 1 teaspoon salt and 1 teaspoon black pepper, then form into 8 balls.
3. Heat vegetable oil in a large skillet over medium-high heat. Add 2 beef balls and press lightly with a metal spatula. Place a handful of sliced ​​onion on each patty (for 2 patties, you'll need about a quarter of an onion—it will seem like a lot!) and flatten the patties to a thickness of 0.5 cm, pressing the onion into each patty. Cook until golden brown on the bottom, about 3 minutes.
4. Carefully flip the patties over, along with the onions, and cook until the onions are golden brown and soft, about 2 minutes more. Flip the patties over again, along with the onions, and top each with a slice of cheese. Cover and continue cooking until the cheese is melted, 30 seconds to 1 minute. Transfer to a baking sheet and keep in a warm oven until ready to assemble the burgers.
5. Fry the remaining patties with onions in the same manner, adding more vegetable oil to the pan if necessary. Serve 2 patties per bun with ketchup, mustard, and/or pickles.