In a bowl, combine cabbage, capsicum, carrot, mayonnaise, salt and pepper. Mix all the ingredients well and set aside for filling.
Cut the brown sides of bread slices with a sharp knife.
Grill the bread slices in a sandwich griller until brown. If you do not have a sandwich griller, you can toast the slices in a non-stick pan.
Take a bread slice and spread 1 tbsp of tomato ketchup on it evenly. Spread the prepared filling on the slice and place slices of cucumber and onions.
Top with another bread slice and spread the filling on it. Also, add sliced tomatoes and cabbage on top.
Take the third bread slice, spread the ketchup and mayonnaise on it and cover the sandwich with this slice.
Cut the sandwich in the shape you like, diagonally or horizontally. Serve the sandwich and enjoy.