These burgers can be cooked either in a pan on the stovetop or using an outdoor grill.
Finely chop the onion and herbs, then put them in a bowl. Add the ground beef, salt and pepper to taste, then mix well until the mixture is well combined and smooth. Using floured hands, form the mixture into flat patties.
Lightly brush the burgers with oil and cook on a hot grill for about 10 min, until the outside is golden brown and the inside loses its pink colour. Turn the burgers once halfway through the cooking.
Alternatively, the burgers can be cooked on the stovetop. Melt a little butter in a frying pan over medium-high heat. Add the burgers and cook for about 10 min, turning them once.
Place each burger on a slice of bread. Add the lettuce leaves and tomato slices. Serve with mustard and ketchup on the side.