Cut the mushrooms into thin slices, chop the onions very finely, press the garlic. Put on the pasta water.
Just before adding the pasta to the water, sauté the onions and garlic in hot oil, add the mushrooms and fry them. Cook the pasta for about 8 minutes until al dente.
Deglaze the mushrooms with the broth, mix the sour cream with tomato paste and add. Dilute the cornstarch with a little water so that there are no lumps and stir into the sauce. Let everything simmer for about 5 minutes.
If necessary, dilute the sauce or, depending on your taste, thicken it again with cornstarch. Season to taste with salt and pepper. Tastes delicious with parmesan and fresh basil.