Prepare: Coat fillets with mustard, place side by side in a suitable container.
Buttermilk pickle: Mix all ingredients, pour over meat.
Cover and refrigerate for 2-3 days.
Nut crust: Remove meat, wipe off pickle. Cut Weggli and walnuts into small pieces in a cutter or chop by hand. Season meat, turn to other side in Weggli nut mixture, pressing well to smooth.
Prepare: Preheat stove and meat plate to 80 °C. Heat oil or clarified butter in pan until hot. Reduce temperature, sear meat on both sides for 3-4 min each, place on platter and cook in stove for about 45 min. If meat cannot be served on the spot then, reduce temperature to 60 °C and keep meat warm for up to 1 hour.
Serve: Cut meat diagonally into pieces, bring to table with pumpkin risotto, for example. (Sauté onions and pumpkin cubes, add long grain rice, gradually add soup, make a risotto).
Tip: Instead of clarified butter, you can also use butter in most cases.