Heat oil in a pan and saute chopped onion. When it becomes soft, add chopped tomatoes, crushed pepper and salt. Stir for a few minutes and add tomato ketchup. Cook till tomatoes become soft. Remove from gas and let it cool. Mash the boiled eggs. Add the mashed eggs and mayonnaise to onion, tomato mix. Combine everything well. Spread on one slice of bread and cover it with another slice.
Healthier version
Use only egg whites, brown bread and low fat mayo and follow the above recipe.
Notes
If you are using tomato sauce, adjust the quantity of tomato accordingly. The taste of crushed pepper is highlighted in this sandwich. In the above picture, I've used only egg white in the filling and for topping I made a paste with egg yolk & mayo.