For the farce, peel and finely dice the shallots. Clean and finely chop the mushrooms. Wash parsley, dab dry and chop as well, except for something to garnish. Heat 20 g fat in a frying pan, fry the mushrooms and onions until there is no more liquid in the pan.
Deglaze with brandy, add parsley and season with salt, pepper and nutmeg. Let it cool down. Place puff pastry sheets next to each other and defrost at room temperature for about 20 minutes.
Heat 2-3 tablespoons of oil in a frying pan and fry the fillet of beef for 10-15 minutes while turning. Season with salt and pepper and let it cool down a bit. Slightly moisten 5 puff pastry sheets with cold water, place them on top of each other and roll out into a square (approx. 35 x 35 cm) on a floured work surface.
Separate the egg and brush the edges of the dough with egg white. Sprinkle breadcrumbs the size of the fillet on the front edge of the dough and add some farce. Place the fillet on top. Spread the rest of the farce on the fillet, press it down and wrap it in the dough.
Press the edges of the dough well. Roll out remaining dough slightly and cut out mushrooms as desired. Brush some mushrooms with egg white and decorate the fillet with it. Place on a baking tray lined with baking paper.
Mix the egg yolk and 1 teaspoon of cream and spread it on the fillet. Bake in the preheated oven (electric oven: 200 °C/ gas: level 3) for 25-30 minutes. Brush the puff pastry mushrooms with egg yolk and spread around the fillet after about 15 minutes.
While the fillet is frying, peel and finely dice the onion. Clean, wash and cut the savoy cabbage into strips. Heat the remaining fat in a pot and fry the onion in it. Add cabbage, braise and season with salt, pepper and nutmeg.
Pour in broth and remaining cream, bring to the boil, cover and stew for 15-20 minutes. Heat the remaining oil in a coated pan and spread the frozen potato balls in it. Fry over medium heat until golden brown, shaking the pan several times.
Remove Filet Wellington from the oven and let it rest for about 10 minutes. Sprinkle the sauce thickener into the savoy cabbage sauce, simmer for about 1 minute and season the savoy cabbage again. Cut the fillet into slices and arrange on plates with creamed savoy cabbage and potato balls.
Serve garnished with puff pastry mushrooms and parsley.