In a skillet, melt butter over medium heat. Add onions to the pan and sauté for 5-7 minutes until they're softened.
Meanwhile, chop the mushrooms into quarters. Add the mushrooms, Dijon, and garlic to the pan and cook for about 10 minutes, stirring occasionally. The mushrooms will shrink and soften and the juices will be released and mostly cooked off by this point.
Reduce to low heat and stir in the sour cream, nutmeg, and salt & pepper. Heat through and serve immediately with some fresh parsley sprinkled over top.