Heat the butter in a frying pan.
Add the chopped onion and sliced mushrooms. Sprinkle with salt and pepper. Cook gently for a minute or two.
Pour in the beaten egg and cook over low heat, stirring (see how to cook the perfect scrambled eggs), until the mixture is barely set. Turn off the heat.
Assemble the sandwich: Lay a slice of bread (or the bottom half of a bun or bagel) on a plate. Cover with trimmed lettuce leaves. Spoon the scrambled egg on top of the lettuce. Lay cheese slices on top of the scrambled egg (the residual heat with partially melt the cheese). Top with alfalfa sprouts. Cover with another slice of bread (or the top half of a bun or bagel).
Serve the sandwich and have a great breakfast.