In a bowl, break the eggs, ensuring there is no shell left, and whisk them with a fork.
Once the mixture is uniform, add the red chili powder, turmeric, black pepper and mix well. Set this mixture aside.
Chop the onion in little squares, approximately 5mm in size, and set aside as well.
Choose a frying pan that can hold 4 slices of bread laying flat and put it on the stove on medium heat and apply the 10 sprays of cooking spray.
Once the pan and cooking spray are warm, add the chopped onion and a little salt; keep on mixing for about 2 minutes, until the onion starts to slightly brown, then lower the flame to a minimum and keep on cooking for about 2-3 minutes, or until the desired consistency of the onion.
Add the chopped coriander to the mixture then quickly execute the next step, as it is crucial.
Add the egg mixture to the pan and spread the onion equally within it; ensure the egg occupies roughly the same surface as the four bread slices, once put in a square shape, will.
Once the bottom part of the omelet starts to settle and cook, take the four slices of bread and apply them on top of the egg, one next to the other in a square shape, leaving only 5 millimeters between each slice on each side. Lightly press the slices of bread, so they grab onto the raw egg and 'become one'.
Once the omelet is cooked through, flip the four slices on the back, so as to have the egg on top, and raise the flame to medium.
Once the bread at the bottom is toasted to liking, switch off the flame.
Add the butter on top of the egg and allow to melt; fold in half, effectively creating one large sandwich composed of two bottom slices and two top slices.
Move to a chopping board and slice between the two top slices, so to obtain two masala egg sandwiches and enjoy!