Swiss mushroom burgers

Ingredients:

• 600 g of ground beef
• Salt and freshly ground pepper
• 2 teaspoons olive oil
• 1 small red onion, cut into thin half rings
• 230 g mushroom mixture, sliced
• 1 tbsp. Worcestershire sauce
• 1/4 cup ketchup
• 2 teaspoons Dijon mustard
• 4 slices Gruyere cheese
• 4 sesame seed buns, halved and toasted
• Lettuce leaves
• Chips and pickles, for serving (optional)

Preparation:

1. In a bowl, combine the ground meat with 1 teaspoon of salt and 1/2 teaspoon of pepper. Form the ground meat into 4 round patties, about 2 cm thick. Heat the olive oil in a frying pan. Add the patties and fry over medium heat until cooked through, about 4 minutes per side.
2. Heat 2 teaspoons olive oil in a separate skillet over medium-high heat. Add the onion and cook, stirring, for 2 minutes. Add the mushrooms and cook for another 2 minutes. Stir in the Worcestershire sauce and cook until tender, about 2 minutes more. Season with salt and pepper. Combine the ketchup and mustard in a bowl.
3. Sprinkle the patties with the mushroom mixture and top with cheese. Return the patties to the pan, cover, and cook until the cheese melts, about 1 minute.
4. Spread the buns with the ketchup mixture, place the mushroom and cheese patties on the bottom halves, then top with lettuce leaves, and top with the top halves of the buns. Serve the Swiss burgers with chips and pickles.