Preheat oven to 400. Line baking sheet with heavy duty foil, then parchment paper. (The foil makes it so you don't have to wash the pan and the parchment paper prevents the meatballs from sticking to the foil.)
Combine all ingredients for meatballs. Using a small cookie scoop, form meatballs and bake on baking sheet for 15 minutes.
While that is baking, put oil in skillet and dice onion. Begin cooking the onions while you slice the mushrooms. Add to onions.
When onions and mushrooms are nearly done, add beef base, black pepper and garlic powder. Add 2 cup of water to the pan. Bring to a boil. Combine cornstarch with remaining 1/2 cup of water. Gradually add cornstarch mix to pan and stir as you are pouring. Cook, stirring often, until thoroughly mixed.
Add cooked meatballs to mushroom sauce, cover with lid and bring to a simmer. Simmer for 30 minutes.
Note that the meatballs will grow when cooking because of the raw rice in them.
Nutrition info on 5 meatballs with sauce: 258 calories, 11 g fat, 81 mg cholesterol, 581 mg sodium, 193 mg potassium, 20 g carbs, 1 g fiber, 2 g sugar, 20 g protein