Sauté the onion and garlic in the butter. Add the spinach and defrost. Season with salt, sugar and nutmeg.
Divide the fish fillets lengthways, acidify with lemon juice, season with salt and pepper. Roll up.
Put the spinach in a greased baking dish. Put fish rolls on it. Prepare the sauce powder according to the instructions (you can also prepare a bechamel sauce yourself). Alloy with egg yolk and pour over the casserole. Scatter breadcrumbs on top, cover with flakes of butter.
Cook in the preheated oven for 20 minutes at 225 ° C.