Clean the mushrooms and cut them in slices. Peel the onion, chop it finely and sweat it in 1 teaspoon of hot butter. Then add the mushroom slices and let simmer, covered, for about 15 minutes.
Heat the remaining butter and dissolve the Gorgonzola in it. Add sour cream and season with salt and pepper. Add to the mushrooms and let steep for another 5 minutes.