Steps
1. In a bowl, mix the chopped onions with chaat masala, red chili powder, cumin powder, and a pinch of salt.
2. Spread ketchup on one side of each bread slice.
3. On one slice, spread mayonnaise over the ketchup.
4. Layer the spiced onions evenly over the mayonnaise.
5. Sprinkle the grated cheese on top of the onions.
6. Cover with the second slice of bread, ketchup side down.
7. Heat a pan and add butter.
8. Grill the sandwich on medium heat, pressing gently, until both sides are golden brown and the cheese melts (about 3-4 minutes per side).
9. Remove from pan, cut diagonally, garnish with fresh coriander if desired, and serve hot.