Heat the olive oil in a frying pan, add the onion and cook for 5 minutes until softened and starting to turn golden. Set aside.
In a bowl, combine the beef mince with the herbs and the egg. Season, add the onions and mix well. Using your hands, shape into 4 patties.
Cook the burgers on a preheated barbecue or griddle for 5-6 minutes on each side. While the second side is cooking, lay a slice of cheese on top to melt slightly (if using).
Meanwhile, lightly toast the cut-sides of the buns on the barbecue. Fill with the lettuce, burgers and tomato slices. Serve with ketchup, if you like.