Wash the plaice and dab dry. Sprinkle with 3 teaspoons of lemon juice and season with salt and pepper. Turn the plaice in the flour (knock off excess flour). Heat clarified butter in 2 large, coated pans and fry the plaice first on the light side until golden brown, then turn and fry the dark side. In the meantime peel the onions and cut them into rings. Wash parsley, dab dry and chop finely.
Remove the plaice and place on preheated plates. Fry onion rings in hot frying fat until transparent. Add crabs and parsley, warm up briefly and season with salt, pepper and remaining lemon juice. Arrange on the plaice. Garnish with lemon and parsley. Serve with buttered potatoes