Decide what you are going to this over, such as egg noodles, mashed potatoes, or rice and get that started so it is ready when you finish preparing the stroganoff.
Open the package of meatballs, drain if needed, then add to a saucepan. Add water to cover by 2 inches. Place on high heat and bring to a boil, continue to boil for 1-2 minutes. Drain. (Photo shows the meatballs cooked and just before draining.)
Heat 1 tablespoon olive oil in a large nonstick pan, when hot, add the meatballs. Sauté, stirring often to lightly brown the meatballs. When they are browned, remove with a slotted spoon to a bowl and set them aside.
Same pan used to brown the meatballs, heat the remaining tablespoon of olive oil and the butter, when hot, add the mushrooms and garlic. Sauté for 4-5 minutes to soften the mushrooms, stirring them often.
Stir in the flour and cook for 1 minute.
Add the soup, water, and mustard, stir to mix together.
Add the meatballs back to the pan and stir in. Increase heat and allow mixture to come to a boil, then reduce heat and simmer for 5-10 minutes to thicken the sauce.
Remove from heat, stir in the sour cream, taste and season with salt and pepper as desired.
Serve over hot cooked egg noodles, mashed potatoes, or rice, enjoy.