Beat the puree with half the butter, season with salt and pepper. Pour into a piping bag and use it to pipe a border on a large heatproof serving platter.
In a skillet, heat bacon fat and remaining butter, add mushrooms, fry for a few minutes and season with salt and pepper.
Place on the center of the plaice fillets, roll them up from the tail end and pin them together with a wooden skewer. Now spread them side by side on the plate.
Mix the cream with the grained broth, season with salt and pepper, then pour over the plaice fillets. Preheat the oven to 180 degrees. Cover the plate with aluminum foil, taking care not to crush the sprayed potato edge.
Cook the plaice on the middle rack for 15 minutes. Carefully remove the foil and wooden skewers and garnish with the parsley. Serve immediately.