Thaw the meatballs in the microwave.
In a skillet, saute the onion and garlic in a bit of oil. Add in the carrots and cook for a few minutes.
In a bowl, combine the pineapple juice with enough water to make 1 1/2 cups. Stir in the vinegar, brown sugar, soy sauce and cornstarch. Add to the skillet and stir till thickened.
Add the meatballs and pineapple.
Simmer until the meatballs are hot and the carrots are tender-crisp.
Serve over rice.