Blanch the savoy cabbage leaves in boiling salted water, rinse in cold water, remove the central rib, then clean the mushrooms (rub with kitchen paper) and chop finely. Mix with grated Gruyere, an egg, salt and pepper.
Lay out the savoy cabbage leaves, spread thinly with the mixture (2 pieces next to each other), place the fish on top and fold in the sides, stir the savoy cabbage packets in a non-stick pan with a little butter, pour 100 ml white wine and 200 ml cream in a tube at 180 degrees Cook for about 20 minutes.