Wash the fillet, dab dry. Heat the oil in a pan and fry the meat on all sides, turning it over. Season with salt and pepper. Remove and place in an ovenproof dish. Cook in a preheated oven (electric cooker: 100 °C/ convection oven: not suitable/ gas: not suitable) for approx. 1 1/2 hours. (core temperature 61 °C)
Knead the flour and 2 tablespoons of butter, divide into small flakes and chill. Wash and clean the mangetouts. Wash the asparagus, cut off the lower ends, cut asparagus into pieces. Peel and finely chop the onion. Remove the fillet, cover with aluminium foil and let it rest for a short time. Cook pasta in boiling salted water according to package instructions. Add the last approx. 5 minutes of asparagus, the last approx. 2 minutes of mangetouts. Wash the herbs and dab dry. Remove leaves and chop finely, except for a little bit for garnishing.
Sauté onion in the frying fat. Deglaze with cream and beef stock, bring to the boil. Gradually fold in flour butter until the sauce thickens. Stir in herbs. Season to taste with Noilly Prat, salt and pepper
Drain the pasta, let it drain. Melt 1 tablespoon butter and toss the noodle mixture in it. Mix in some sauce. Serve fillet with noodles and remaining herb sauce. Garnish with chervil