In a mixing bowl add the chopped onions, capsicum and cucumber.
Next, add a ¼ cup of mayonnaise along with black pepper and salt according to your taste.
Mix all the ingredients well, in way that veggies are fully covered in mayonnaise.
Take the bread slices and apply butter on both slices.
Spread the veg mixture evenly on to each bread slices and cover it with another bread slice on top.
Place the bread slices in the grill machine and cook until the bread slices turn into light brown colour. You can make these sandwiches on frying pan also.
Once you get your desired colour, switch off the machine.
Cut the sandwich into a triangle shape and enjoy delicious veg mayonnaise sandwich with a cup of tea or like itself.