Cook ground beef and diced onion in a 10-inch skillet over medium heat, stirring occasionally. Add mushrooms after 5 minutes and cover with a lid. Simmer until the mushrooms are tender. About 8 minutes.
Add cream of mushroom soup and mustard. Continue to simmer until heated through. Remove the skillet from the heat source and stir in sour cream.
Serve warm over egg noodles, spaetzle, rice, or mashed potatoes.
Notes
TIP: Mushrooms will lose a lot of liquid while they are simmering. If you find that there is too much liquid and you are worried that your Stroganoff sauce will be too thin, remove some with a large spoon.You can sauté the mushrooms separately in a different skillet which is more work. I don't recommend it for this recipe as it defies the purpose of making a truly simple and delicious comfort food recipe.