Toast your bread in the toaster or oven. Whisk the egg with a dash of milk, salt and pepper. Grease a small frying pan with a little butter and heat over medium heat. Once hot, pour the egg mixture into the pan and let it cook for about 3 minutes.
Flip one half over and continue to cook until desired doneness. Let cool. Butter both slices of your bread generously. Stack some lettuce and tomato on one buttered slice and put your omelette on top. Top it off with the other slice of bread. Use a toothpick (or several) to keep the sandwich in place.