Animal Style Hamburgers

Ingredients:

• 2 tablespoons vegetable oil, plus extra for buns
• 2 large onions, finely diced
• 1/4 cup mayonnaise
• 2 tbsp. ketchup
• 3/4 cup chopped iceberg lettuce
• 1/2 teaspoon of 5% table vinegar
• 900 g lean ground beef or veal
• 4 hamburger buns, cut in half
• 1/4 cup sliced pickled cucumbers
• 1 tbsp sweet vegetable relish (heat-processed pickled chopped vegetables or fruits)
  recipe corn relish
• 4 - 8 thin slices of tomato
• Coarse salt and freshly ground pepper
• 1/4 cup mild yellow mustard
• 8 slices American cheese or any other semi-hard cow's milk cheese

Preparation:

1. Heat vegetable oil in a frying pan to medium-low temperature.
   
   Add the onion and 3/4 teaspoon salt, cover and cook, stirring occasionally, until softened and golden brown, about 30 minutes. (If the onion is browning too quickly, reduce the heat.)
   
   Remove the lid, increase the heat to medium, and continue cooking, stirring frequently, for about 8 minutes, until the onions are caramelized. Add 1/2 cup water to the pan and bring to a boil. Continue cooking until the water has evaporated slightly, about 2 minutes more, then transfer to a bowl.
   
   Caramelized onions can be refrigerated for up to 3 days, reheating before using.
2. In a separate bowl, mix the mayonnaise, ketchup, relish, and vinegar. Form the minced meat into 8 round patties, approximately 10 cm in diameter and about 1.5 cm thick.
3. Heat a frying pan to medium heat; lightly grease with vegetable oil. Place the buns, cut side down, in the pan and fry until golden brown.
   
   Spread 1 tablespoon of the mayonnaise mixture on each bottom bun half, then top with a few cucumber slices, a small amount of lettuce, and 1-2 tomato slices. Top with the remaining mayonnaise mixture.
4. Season the patties with salt and pepper on each side. Place them in a hot frying pan and fry for 3 minutes. Then spread 1.5 teaspoons of mustard on each patty, flip them over, and top with 1 slice of cheese. Continue cooking for about 2 more minutes (the patties will be medium-rare).
5. Top 4 patties with caramelized onions, then top with the remaining patties, cheese-side up. Place the double patties on the vegetable buns, then top with the top halves of the buns.