Heat a medium nonstick skillet over medium heat. Add the bacon and cook, 5 to 6 minutes, turning halfway until crisp. When cooked, set aside on a paper towel and wipe some of the fat, leaving a little to cook the egg. Crack the egg, cover and cook until the yolk is set.
Meanwhile, toast the bread. Assemble the sandwich by putting the mayo on the bottom, then the egg, then the lettuce, bacon and tomatoes. Cut in half and eat right away.