Wash and peel the potatoes and cook them in little boiling salted water for about 20 minutes. Clean and wash spring onions and cut into fine rings. Wash the plaice, dab dry, sprinkle with lemon juice and season with salt and pepper.
Put the oil in two large coated pans and heat them up. Place the plaice in them and fry on each side for about 7 minutes. In the last 2 minutes add 10 g roasted onion butter to the plaice. Arrange the plaice on preheated plates.
Heat the rest of the onion butter in a frying pan and sauté the spring onions briefly. Add the crabs, toss them and season with salt and pepper. Spread the crab and onion mixture over the plaice and add the boiled potato.
Serve garnished with dill and lemon wedges.