Melt 4 tablespoons butter in a large non-stick pan over medium heat. Add the mushrooms, onions, 1/2 teaspoon salt, and 1/2 teaspoon pepper and cook them for 10 to 15 minutes or until the mushrooms have released their moisture. Add the garlic and cook it with the mushrooms and onions for a minute or so.
Sprinkle 2 tablespoons flour over the top of the mushrooms and stir to coat them. Add 1/2 cup white wine and stir. As it simmers, scrape up all of the browned bits off the bottom of the pan.
Add 1 cup beef stock and 2 tablespoons mustard. Stir and then add 24 ounces frozen meatballs. Cover the pan and adjust the heat to medium. Simmer the meatballs for ten minutes or until they are warmed through.
Uncover the pan and take it off the heat. Add 1/4 cup sour cream and stir to combine.
Serve the meatball stroganoff with noodles or rice. Garnish with chopped parsley or chives.
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Notes
The white wine adds acidity which balances the richness of the sauce. If you prefer not to use it, use the same amount of beef stock (so 1 1/2 cups total).If you're not a big fan of mustard, reduce the amount to 1 tablespoon. You can always add more after you add the sour cream.