Melt the butter in a sauté pan over medium heat. Add the mushrooms, onion and salt and saut´, stirring, for 10 minutes. Stir in the sherry or vegetable broth and reduce heat to low. Slowly sprinkle in the flour and stir to dissolve. Keep stirring for an additional minute or two, until all of the flour has been absorbed. Add the garlic and sprinkle with pepper to taste. Continue to cook, stirring, over low heat for 5-10 minutes, until all ingredients are heated through and the flavors begin to fuse together. Stir the vegetable or mushroom broth into the sauce, then fold in the sour cream or yogurt. Mix well until it is completely incorporated and heated through.

While the sauce is simmering, cook the pasta in boiling water according to package directions and drain.

Serve over hot cooked pasta and top with freshly grated Parmesan cheese!