Put 2 quarts of water, salted with 1 tablespoon of salt, in a pot to boil while you cut the mushrooms and onions.

Heat a large sauté pan on medium high heat. Add the mushrooms to the pan and dry sauté them until they begin to release their water. Sprinkle them with a little salt. Once they have released most of their water, remove them to a bowl and set aside.

Add the green beans to the boiling water, and time them for 5 minutes, or until just tender. (No need to defrost frozen beans.)

Add butter to the hot sauté pan. Once the butter has melted, add the onions. Cook until the onions are beginning to brown.

As soon as the green beans are cooked just tender, strain them. Add the mushrooms to the pan with the onions, then add the green beans.

Remove from heat. Stir in the sour cream. Sprinkle with salt and freshly ground black pepper. Taste and add more salt if needed. Sprinkle with parsley and serve.