For the zucchini fish rolls, dice the rolls and crumble finely in a food processor. Roast mustard seeds in a pan without fat and let cool. Pluck dill tips, chop finely and mix with mustard, créme fraîche, mustard seeds and breadcrumbs. Season with salt, pepper and sugar and set aside.
Clean zucchini and slice lengthwise into 16 slices (about 5 mm thick), divide into perforated cooking containers and pre-cook until pliable (3-4 minutes at 100°C).
Wash fish fillet, pat dry and cut into strips 3 cm wide and long enough to match the zucchini slices. Salt and pepper the zucchini slices and the fish. Spread each zucchini slice with mustard cream, cover with a fish strip and roll up. Pin with wooden skewers and place upright in perforated steamer basket. Place a non-perforated cooking container underneath and cook the fish (at 100°C for 8 minutes).