Wash the lettuce leaves and dab dry. Wash the cucumber and cut into thin slices. Wash the tomatoes and cut them into slices as well.
Finely crumble cornflakes. Whisk the egg in a deep dish. Mix flour, 1 level tsp salt and pepper in a deep plate. Dab meat dry. First turn in flour, then in egg and finally in cornflakes.
Heat the oil in a large frying pan. Fry the meat on each side for 4-5 minutes at low to medium heat.
Meanwhile, toast bread in the toaster. Remove the schnitzel from the pan. Cover 4 slices of bread with 1 lettuce leaf, 1 escalope, 1 tablespoon ketchup, cucumber and tomato. Season lightly with salt and pepper. Place the remaining slices of bread on top and cut the sandwiches diagonally.