In a bowl, mix chicken, yogurt, paprika, tikka masala, lemon juice, ginger garlic paste, and salt until the chicken is fully coated.
Heat oil in a pan over medium heat, add the marinated chicken, and cook for 8–10 minutes until tender, then raise the heat to dry up any remaining liquid.
Let the chicken cool slightly, then roughly chop it into small, bite-sized pieces and set aside.
In a small bowl, whisk the egg with salt and black pepper.
Heat oil in a pan and fry the egg mixture like a flat omelet, cooking both sides until lightly golden, about 1–2 minutes total.
Trim the crusts off your bread if you want a cleaner look, then toast the slices in a buttered pan until both sides are golden and crisp.
On one slice, spread a mix of mayonnaise and ketchup, then layer with lettuce, cucumber slices, the omelet, and a slice of cheese.
Top with a second toasted slice, then add the chopped chicken filling generously.
Finish with the third slice of bread, press gently, cut the sandwich into halves or quarters, and insert a skewer to hold it together if you like.