Preheat the oven to 85°C. Heat up a fat pan and an oven rack. Rub the fillet of beef with salt and pepper. Heat 3 tablespoons of oil in a roaster. Brown the fillet in it all around. Take it out and put it on the preheated grid in the oven. Slide the fat pan underneath. Cook beef fillet for approx. 4 hours
Clean the carrots, leaving about 2 cm of green. Wash the mangetouts, remove the side threads. Simmer carrots in boiling salted water with 1 teaspoon sugar for 10-15 minutes. Blanch the sugar snap peas in salted water with 1 teaspoon sugar for about 4 minutes. Rinse vegetables with cold water, put aside
For the Rösti, peel the potatoes and grate into fine strips. Season with salt, pepper and nutmeg. Cover and allow to stand for a while
For the tarragon sauce, peel and finely chop the shallots and garlic. Melt 1 tablespoon butter and 1 tablespoon oil in a saucepan. Sauté the shallots and garlic in it until translucent, but do not let it get brown. Add tarragon and 500 ml wine. Bring to the boil over high heat. Let the liquid boil down almost completely. Add cream and double cream. Reduce while stirring over medium heat.
Drain the potatoes, squeeze a little. Heat 3 tablespoons of oil in portions in a frying pan. Add the grated potatoes bit by bit with a small ladle and press down gently. Fry the Rösti over medium heat on both sides until golden brown, drain on kitchen paper. Keep warm
Take the fillet out of the oven, wrap in aluminium foil and let it rest. Remove the roasting set with 50 ml wine and broth, add to the sauce. Simmer gently for about 5 minutes. Add 100 g cold butter in cubes to the sauce. Mix with a hand blender. Heat the carrots and sugar snap peas in a saucepan with 1 tbsp. butter and a little sugar each. Serve everything