Bring two liters of water with 2 teaspoons of salt to the boil, put in the pasta, stir and allow to soak in the residual heat for about 10 minutes, then drain and drain.
For the sauce, peel the onions and cut them into cubes. Heat the butter and sauté the onion cubes. Clean the boletus, cut into slices, add to the onions and sauté.
Deglaze with the beef stock and cream, bring to the boil and continue cooking until the sauce is creamy.
Season the sauce with salt and pepper.
Mix the flour with 6 tablespoons of water, stir into the boiling sauce and let it soak for 5 minutes in the residual heat.
Arrange the ribbon noodles on plates and distribute the sauce on top.