Double-patty cheeseburger

Ingredients:

• 2 tbsp vegetable oil + extra for greasing the grill
• 2 large onions, finely chopped
• 1/4 cup mayonnaise
• 2 tbsp. ketchup
• 1 tbsp pickled cucumber relish
• 0.5 tsp white vinegar
• 1 kg of ground beef (preferably 20%-40% fat)
• 4 hamburger buns, cut in half horizontally
• 1/4 cup sliced pickled cucumbers
• 4-8 thin slices of tomatoes
• 3/4 tbsp. torn iceberg lettuce
• 1/4 cup mustard
• 8 slices of cheese

Preparation:

1. Heat vegetable oil in a large skillet over medium-low heat. Add the onion and sprinkle with 3/4 teaspoon salt. Cover and sauté, stirring occasionally, until golden brown and softened, about 30 minutes. If the onion is browning too quickly, reduce the heat to low. Remove the lid, increase the heat to medium, and continue cooking, stirring frequently, until caramelized, about 8 minutes. Pour in 1/2 cup water and scrape up any browned bits from the bottom of the skillet. Cook, stirring occasionally, until the water evaporates, about 2 minutes; transfer the onion to a bowl.
2. In a small bowl, combine mayonnaise, ketchup, relish, and vinegar. Form the ground beef into 8 patties, each about 10 cm in diameter and about 1 cm thick.
3. Heat a large skillet over medium heat and lightly grease with vegetable oil. Toast the buns cut-side down. Spread about 1 tablespoon of the mayonnaise mixture on the bottom half of each bun, then top with a few slices of pickle, 1-2 tomato slices, some lettuce, and another spoonful of mayonnaise mixture. Keep the skillet on the heat.
4. Season the patties on both sides with salt and black pepper. Working in batches, transfer them to the skillet and cook for 3 minutes. Spread about 1.5 teaspoons of mustard on the surface of each patty, then flip and top with 1 slice of cheese; cook for another 2 minutes for medium-rare. Top 4 patties with caramelized onions, then top with the remaining patties, cheese side up. Transfer the double patties to buns; top the burgers with the bun tops.