Place beef in a mixing bowl, and push to one side of it. Add rice and 1 1/2 cups of stock in the other side.
Add salt and pepper, and let rice absorb stock for about 1 minute.
With an electric mixer, mix for about 1 minute until both sides are homogenous.
Form the mixture into 16 equal-size meatballs.
Coat a large skillet with olive oil cooking spray and place over medium heat. Once the skillet is hot, add the meatballs and brown on one side, about 1 minute. Turn the meatballs over and brown the opposite sides, about 30 seconds.
Transfer meatballs into holding plate.
Spray skillet again with oil. Add the mushrooms and onion, and cook for 3-4 minutes, until brown and softened. Add spinach, and cook until wilted, about 1 minute.
Add remaining stock and arrowroot slurry. Whisk until thickened.
Add meatballs and simmer until cooked through, 6-8 minutes. Spoon the meatballs and mushroom mixture into four bowls, distributing them equally. Sprinkle with cheese, and serve. Enjoy!
*Tip: Include some no-sugar-added dried cranberries if you want to liven things up a bit for an additional 20 calories per tablespoon.