Add the meatballs and onions to the bowl of a slow cooker, then stir to combine.
1 3 lb bag frozen meatballs, 1 large yellow onion
In a mixing bowl add the remaining ingredients, except for the cornstarch, then whisk together until evenly combined. Pour the sauce evenly out over the meatballs, then stir to combine.
2 10.75 oz cans condensed beefy mushroom soup, 1 pkt onion mushroom soup mix, 1 1/2 cups beef broth, 1/4 cup A1 steak sauce
Cover & cook on HIGH for 3-4 hours.
At the last 30 minutes of cook time, removed 1/4 cup of the juice from the crockpot and pour it into a small bowl. Pour the cornstarch into the bowl, and whisk together until smooth and evenly combined. Add the mixture back into the crockpot, stirring to combine. Cover, and continue cooking.
2 tbsp cornstarch
Serve warm over a bed of mashed potatoes, or cooked egg noodles.