Wash and thinly slice the tomato, cucumber, onion, and capsicum so they layer evenly inside the sandwich.

Blend the mint leaves, coriander leaves, green chili, and salt with a splash of water into a smooth, spreadable green chutney.

Mash the boiled potato in a bowl and mix in chaat masala, red chili powder, and salt until evenly combined.

Spread the mint chutney on one bread slice, layer on the mashed potato and sliced vegetables, then top with a second bread slice.

Spread a thin layer of butter on the outer sides of both bread slices to help them turn golden and crisp on the grill.

Turn on the sandwich grill or heat a flat tawa and let it come up to temperature before placing the sandwich on it.

Place the assembled sandwich on the hot grill and close the lid, letting the first side cook until lightly marked.

Continue grilling, flipping if using a tawa, until both sides of the sandwich are evenly golden-brown and crisp.

Let the grilled sandwich sit for a moment off the heat so the filling settles and it holds together when cut.

Cut the sandwich diagonally into triangles and serve warm alongside the remaining mint chutney.