Place egg in water and let cook until soft boiled (6-7 minutes NOTE: No more or it will be hard boiled).
Meanwhile, place cheese slices on each side of bagel and layer with tomato and onion. Toast in toaster oven (or regular oven at 400F) until cheese is melted (about 3-5 minutes).
Top with avocado, peel and break the soft boiled egg over the sandwich and top with other bagel half.