Mini beef sandwiches for a crowd

Ingredients:

• 1 medium onion
• 700 g of ground beef with 10% fat content
• 1 large egg
• 1/4 cup plain breadcrumbs
• 2 teaspoons of vegetable oil
• 6 slices American cheese, quartered
• 24 thin slices of plum tomatoes (2-3 tomatoes)
• 24 slices of pickled cucumbers
• 24 mini potato buns
• 4 large iceberg lettuce leaves, cut into 24 equal pieces
• Ketchup and mustard for serving
• Special equipment: baking tray with sides measuring 25x37.5 cm.

Preparation:

1. Preheat the oven to 220°C. Cut the onion in half and grate each half coarsely. Squeeze the liquid out of the grated onion and scatter it over the entire surface of a 25 x 37.5 cm baking sheet.
2. Place the ground beef, egg, breadcrumbs, and 1 teaspoon of salt in a large bowl and gently toss to distribute the ingredients evenly. Mound the meat mixture over the entire baking sheet and press it into an even layer, being careful not to disturb the onions (it's okay if some are visible). Brush the surface of the meat with vegetable oil, then evenly sprinkle with 0.5 teaspoon of salt and a little freshly ground black pepper. Using a spatula, divide the meat into 24 squares.
3. Bake until the meat is browned, 8-9 minutes. Place a small square of cheese on each patty and return to the oven to melt it, about 30 seconds. Let it rest for a few minutes, then run a spatula between the patties to separate them.
4. Top each cutlet with a slice of tomato and pickle. Leave a spatula on the baking sheet so guests can help themselves. Place a plate of buns and lettuce leaves on the table nearby. Don't forget the ketchup and mustard.