Instructions

1. 1
   Heat a thin layer of vegetable oil in a frying pan. Add the onion, and fry for about 5 minutes until softened. Remove from the heat and cool.
2. 2
   Combine the ground beef, egg, and cooked onion.
3. 3
   Shape the beef mixture into patties. Cook the burgers on a grill for about 6 minutes, flipping halfway until browned on each side and cooked through.
4. 4
   In the last minute of cooking, lay a slice of cheese on top of each patty. Cover and let the cheese melt slightly.
5. 5
   Lightly toast the hamburger buns. Sandwich the patties in the buns, topping with lettuce, tomato, and ketchup as desired.