Finely chop 1 small onion, 1 small capsicum, 1 small carrot, and 1 small cucumber.
In a mixing bowl, add the chopped vegetables, 200 grams of mayonnaise, 1/4 teaspoon black pepper powder, and salt to taste. Mix well to combine all the ingredients.
Take 8 slices of bread (preferably whole wheat or multigrain) and spread a generous amount of the veg-mayonnaise mixture on one side of each slice.
Place another slice of bread on top of the mixture to make a sandwich. Repeat this process for the remaining slices.
Heat a non-stick pan or sandwich maker. Apply a thin layer of butter on both sides of each sandwich.
Toast the sandwiches on medium heat until they turn golden brown and crispy on both sides.
Cut the sandwiches into halves or quarters and serve immediately with ketchup or chutney.