In a big Bowl, mix onion, capsicum, tomato, carrot, cucumber, green chillies, green coriander, mix well.
Add curd, black pepper, salt and mix well.
Filling is ready, keep aside.
Apply butter on 1 side of each slice.
Heat a tava, place 1 slice on tava (buttered side on tava), spread ¼th of filling on the bread slice, cover it with another slice (buttered side on top).
Cook on low to medium heat, by pressing with wooden spatula (bread slice will stick to the filling nicely).
Once golden brown from 1 side, flip and cook the other side also to golden brown.
Vegetable sandwich is ready.
Similarly, make rest of the sandwiches.
Serve hot with coriander chutney or sauce.