For the dough, briefly mix 175 g flour, 1⁄4 TL salt, 125 g cold butter in pieces and cream cheese with the dough hooks of the mixer. Knead in 3 teaspoons of ice-cold water with a fork. Knead onto some flour and wrap in foil and refrigerate for about 2 hours.
For the filling, wash the fillet. Cook in approx. 3⁄4 l boiling salted water for approx. 15 minutes. Lift out, measure approx. 1⁄4 l stock for the sauce. Peel onion, clean and wash paprika and mushrooms. Cut everything finely. Heat 3 tablespoons of butter. Fry onion and mushrooms. Fry the bell peppers briefly. Pluck the meat and mix with vegetables and herbs. Season with salt and pepper.
For the sauce, heat 5 tablespoons of butter, lightly sauté 45 g flour in it. Stir in milk and 1⁄4 l stock, bring to the boil and simmer for about 3 minutes, stirring continuously. Season with salt, pepper and nutmeg. Mix with the filling. Season to taste. Pour into a round casserole dish (approx. 24 cm Ø).
Roll out the dough on flour to a thickness of approx. 5 mm round (approx. 26 cm Ø), place on the form, cut 4 slits. Brush with whisked egg. Bake in a hot oven (electric cooker: 200 °C/circulating air: 175 °C/gas: see manufacturer) for approx. 45 minutes.