Heat up the butter in a big pan over medium-high heat then add the onions and cook for 2-3 minutes until translucent.
Add the mushrooms and cook for another 4-5 minutes until lightly browned and most of the water is dissolved.
Place the meatballs and mushroom and onion mixture in the slow cooker.
In measuring cup or bowl mix together the beef broth, tomato past and mustard.
Add all ingredients EXCEPT sour cream, cornstarch and water to crock pot, stir and cook on low for 6 hours or on high for 3 hours.
Once cooked mix the cornstarch with the water until creamy then add in sour cream and mix until smooth.
Pour mixture into the crock pot, stir and cook for another 30 minutes on high until sauce is thick and creamy
Serve hot over noodles, mashed potatoes or any side dish of your choice.
You can easily double this recipe by using a larger crock pot.
For a quick swap, use canned mushrooms in place of fresh mushrooms in this recipe.
The Homestyle or Swedish flavor of meatballs work best for this.
We usually serve these meatballs over noodles, rice or mashed potatoes.
For some additional spice you can stir in some paprika or cayenne pepper.
Heavy cream can be substituted for the sour cream.
The Crock Pot Meatball Stroganoff Recipe will stay good for up to 4 days covered in your refrigerator. To reheat place in a microwave or pot on the stove. You might need to add a splash of milk. I would recommend not mixing it with the pasta because it will taste better and is easier to warm up when kept separated. The noodles will soak up the sauce and make the dish very thick and heavy.