In the same frying pan used for cooking the meatballs, add finely chopped onions and minced garlic. Sauté over medium heat until the onions become translucent and fragrant, approximately 2-3 minutes. Next, add chopped tomatoes, fish sauce, sugar, and water to the pan. Stir well to combine all ingredients and bring the mixture to a gentle simmer. Reduce the heat to low and continue cooking, uncovered, for an additional 5-7 minutes, or until the tomatoes have softened and the sauce has slightly thickened.