Heat the oil in a pan, sweat the onion in it until translucent. Add the mushrooms and sauté. Pour off the resulting liquid. Deglaze with the white wine and let it boil.
Now add the whipped cream. Season with salt and pepper and simmer for about 10 minutes.
Then add the parmesan, simmer for another 2 minutes and finally fold in the finely chopped parsley. Season to taste again, possibly seasoning again. The sauce goes best with pasta.