Prepare the thecha by adding the peppers to the hot pan and fry on high heat until they are blistered (8-10 minutes). Keep stirring at regular intervals.
Once blistered, remove the chilies on a plate.
Reduce the heat to medium-low and add vegetable oil to the same pan. It will get heated in just a few seconds as the pan is already very hot. Add cumin seeds and garlic cloves and mix everything. Cook until garlic cloves are blistered (2-3 minutes). Keep stirring frequently.
Transfer the cumin seeds and garlic to the plate along with chilies and let everything cool down for 5 minutes.
Add green chilies and garlic mixture to a large mortar and pestle along with salt to taste and pound to make a coarse paste. If you don’t have a mortar and pestle, you can crush the ingredients in a food processor or a chopper. Just make sure to keep it coarse and not pasty.
For the chicken filling, mix chicken with mayonnaise, bell peppers, salt, pepper, Kitchen King masala and red chilli masala together in a bowl.
Lather the thecha on a bread slice, top it with the chicken mix, add slices of tomato and onion. Cover with a buttered bread slice.