Heat 2 tablespoons of the oil in a large non-stick skillet. Add the onion to the pan and cook over medium-high heat, stirring occasionally, for 8-10 minutes, until golden and softened.
Mix together the ground beef, egg, and herbs in a bowl and add salt and pepper to taste. Form into 4 round, flat patties. Brush the bottom of another nonstick pan with the remaining oil and place over high heat. Add the burgers and cook for 3 minutes, then turn over and cook for an additional 2 minutes, until cooked through.
Split the buns and place the sliced tomatoes on the bottom halves. Put the burgers on top of the tomatoes and then add the cooked onions. Serve with lettuce or salad greens.