Drain the pepperoni, remove the seeds and cut finely. Wash the basil and remove the leaves. Slice the Parmesan cheese thinly with a peeler or grate it roughly. Fry the pepperoni and 1 tsp. fennel seed in hot oil.
Wash the fillets if necessary, dab dry. Cut into rectangles like for a roast. Brush meat with mustard. Spread pepperoni, parmesan and basil on top. Roll up the meat and tie up with kitchen string.
Fry the meat in hot clarified butter until golden brown all around. Place on the fat pan of the oven. Sprinkle with the remaining fennel seeds and sea salt. Pour a good 1/8 l water on it. Roast in the preheated oven (electric cooker: 175°C/circulating air: 150°C/gas: level 2) for another 25 minutes.
Let it cool down. Cut fillets into thick slices and arrange.