Preheat the oven (electric cooker: 80 °C/circulating air and gas: not suitable). Dab fillet dry, season with salt and pepper. Heat oil in a pan. Brown the fillet thoroughly all around. Take out and put the pan with the frying set aside.
Place the fillet on a tray and cook in a hot oven for about 3 hours.
3⁄8 Bring l water and 1 teaspoon salt to the boil. Remove from heat and add 1⁄8 l milk. Stir in the puree powder with a whisk, let it cool down.
Wash the rosemary, dab dry and finely chop the needles. Chop nuts as well. Melt 75 g butter. Remove the crusts from the bread. Chop toast and nuts finely in the universal chopper. Stir in liquid butter and 1 egg.
Season with salt and pepper.
Stir 1 egg and flour into the mashed potatoes. Season to taste with salt, pepper and nutmeg. Form small oblong dumplings from the mixture. Cook the frozen vegetables in little boiling salted water for 5-7 minutes.
Quench and drain.
Heat approx. 1 l oil in a wide pot or deep fryer. Bake potato dumplings in it in portions until golden brown. Drain on kitchen paper and keep warm.
Remove the fillet, turn the oven up (electric oven: 225 °C). Spread the fillet with the nut mixture, roast for 10-15 minutes.
Toss the vegetables in 1 tbsp. melted butter, season with pepper and nutmeg. Heat the pan with the roast mixture again. Deglaze with sherry and 1⁄4 l water, bring to the boil. Stir in beef base and boil down for 5 minutes.
Refine the sauce with crème fraîche, season to taste with salt and pepper and foam up with a hand blender just before serving. Cut beef tenderloin into slices. Arrange everything.