Chop the parsley into small pieces. Cut the onions into small cubes. Cut the mozzarella and tomatoes into slices and crumble the rusks.
Alternate the tomato slices with the mozzarella slices on an ovenproof plate. Season with salt and pepper vigorously.
Fry the onion cubes in the butter until translucent. Add the rusk crumbs and chopped parsley and mix everything together. Season to taste with a little salt and pepper. Spread this mixture on the tomato and mozzarella slices.
Place the plate in the preheated oven at 200 ° with the grill switched on (level 2) and gratinate the tomatoes and mozzarella for 10-15 minutes.