In the meantime cut the fillets into fine strips and season with salt. Heat the butter in a large pan and fry the onion cubes until translucent. Add the fish strips and sauté for about 2 minutes while turning over a low heat. Take out and keep warm.
Put the peas in the pan. Pour in the cream and reduce to a creamy consistency over a high heat for 3-4 minutes. In the meantime, finely chop the dill (except for a few twigs for garnish). Add to the sauce with the fish and the diced tomatoes. Season with salt and pepper and let steep for another 1-2 minutes.
Drain the pasta well, serve with the plaice cream and garnish with dill.