Gently mix together ground beef with onion. Form ground beef into 4 equal patties. Make an indentation in the middle of each patty with your thumb. Season patties with salt and pepper.
Heat oil in a large skillet. Add hamburger patties and cook over medium - high to high heat for 3 minutes. Flip patties and cook for an additional 5 minutes.
If adding cheese, add during the last minute of cooking time and cover skillet. Serve immediately on buns with condiments of choice.