Brush the potatoes and cook in boiling water for about 25 minutes. Clean and wash spring onions and cut into fine rings. Wash and halve the cherry tomatoes. Heat a pan and roast the mustard seeds in it while turning over medium heat. Remove the fish carefully from the sauce.
Mix the mustard-cream sauce with part of the spring onions and tomatoes. Drain and break open the potatoes and arrange 2 on each plate. Divide the fish into pieces and put them on the potatoes. Fill the sauce over it and sprinkle with the remaining spring onions, tomatoes and mustard seeds