In a bowl, mix 4 tablespoons of mayonnaise, 1 finely chopped onion, 1 finely chopped tomato (deseeded), 1 finely chopped capsicum, 1/4 teaspoon black pepper powder, and salt to taste. Mix well to combine all the ingredients.
Take 4 slices of bread and spread the mayonnaise mixture evenly on two of the slices. Place the other two slices on top to make a sandwich.
Preheat a non-stick tawa or griddle over medium heat. Apply 1/2 tablespoon of butter on each side of the sandwiches.
Place the sandwiches on the tawa and cook for 2-3 minutes on each side or until they turn golden brown and crispy. Press gently with a spatula to ensure even grilling.
Once grilled, remove the sandwiches from the tawa and cut them into halves or quarters.