Add the egg, salt, mayonnaise, and ground black pepper. Mix all the ingredients well and make sure the egg gets broken up into small pieces.
Toast, grill, or pan-fry 4 pieces of multi-grain bread until the surface of the both sides turns slightly brown and crunchy.
Place 2 slices of bread on the cutting board. Add a few leaves of lettuce to each slice of bread. Divide the tuna mixture into 2 and spread it to each slice of bread. Add radish sprouts and a piece of bread to the top. Cut them into halves and serve.