Mix beer, egg and flour to a smooth dough and season with salt and cayenne pepper. Let stand for 1-2 hours. Grind the crackers in the universal chopper. Peel onions and cut into 1/2-1 cm thick rings. Remove the smallest inner rings and use them for other purposes. Put onion rings in a bowl of cold water for about 1 hour. Wash the tomatoes, dab dry and cut into slices of about 1 cm.
Dab onion rings dry with kitchen paper. Heat oil in a pot. Turn the onion rings in the baking dough and then in the ground crackers and bake in portions in the hot oil until golden brown. Drain well on kitchen paper. Drizzle with a little vinegar. Bake tomato slices like the onion rings and drain. Arrange on a plate together with a small bowl of crème-fraîche and serve garnished with parsley