Heat oil in a pan. Sauté chopped onions till soft and translucent.
Add the chicken pieces; season with salt and pepper. Add a dash of tomato sauce and mix well to combine.
Beat the egg well and pour over this mixture. Cook like a regular omelette flipping over if necessary. Alternately, you can remove the chicken mixture and cook with the egg in batches if you have a smaller pan.
To assemble the sandwich, halve the buns and spread tomato sauce and mayonnaise on either side. Place a few greens on the bottom half followed by slices of tomato, egg chicken omelette and cucumber.
Enjoy