Divide ground beef into 4 equal portions. With a light touch, form beef into patties about the size of your buns.
Season both sides of the patties generously with salt and pepper. Use your fingertips to make a dent at the centre of the burger (just on one side).
Heat a flat heavy based pan over medium-high heat and add 1 tbsp oil. Cook the onion rings until caramelised. Season with salt and pepper and transfer to a plate.
Heat 1 tbsp oil to the same pan. Add the burger patties and cook until crust is formed on the bottom, 2 - 3 mins. Don’t be tempted to press the patties.
Flip the patties over and cook for 1-2 mins, depending on your desired doneness. Top with cheese and cover the pan with lid. Wait until cheese melts, 30 sec - 1 min.
Toast the buns while waiting for the cheese to melt.
Make the hamburger: Spread sauce or condiment on base of the buns. Add lettuce, tomato, then burger patty. Top with onions, pickles, and more sauce or condiments of choice. Put back lid of bun. Serve hot.