Rinse the pikeperch under cold running water, pat dry, acidify, salt and rub with pepper. Melt some of the butter in a baking dish, put the pikeperch in it. Peel and dice shallots. Clean the mushrooms, rub them with kitchen paper, rinse them if necessary and cut them into slices. Add both ingredients, pour the rest of the butter on top. Add white wine. Put the casserole dish on the wire rack in the preheated oven and bake at 170 ° C for about 30 minutes.
For the wine and cream sauce, reduce the fish stock and cream a little. Mix the flour with 5 tablespoons of white wine, add to the stock and cook for 2 - 3 minutes. Stir in the mustard and season the sauce with salt and pepper.
Take the pikeperch out of the baking dish and serve with the sauce.