Wash the asparagus, cut the woody ends off generously. Cut the asparagus into pieces. Cover with a damp tea towel. Peel and finely chop the onions.
Dab the fillet dry and cut into thin slices. Heat the oil in a large pan. Fry the meat in portions on each side for about 1 minute at high heat. Only the bottom of the pan should be covered at all times so that the meat is nicely browned.
Season each with salt and pepper.
Meanwhile cook the asparagus covered in 400 ml boiling salted water with 1 pinch of sugar for 6-8 minutes.
Cook the pasta in boiling salted water according to package instructions.
Drain the asparagus and collect the vegetable water. Sauté the onions in the frying fat. Dust with flour and briefly sauté lightly. Add wine and vegetable water, bring to the boil and simmer for about 5 minutes. Stir in mascarpone.
Season to taste with salt and pepper. Heat the fillet and asparagus briefly in the sauce. Drain the pasta. Arrange everything.