1. In a medium-large bowl, combine the ground beef, chopped onion, rice, water, salt and pepper.
2. Use a spoon to mix the ingredients around a bit, then dive on in with your hands - it really is the best way to mix these up.
3. Form mixture into 12-16 meatballs, roughly the size of a golf ball.
4. In a large skillet, combine ketchup, mustard, brown sugar, stock, and thyme over medium-high heat.
5. Stir well to combine (a silicone whisk would work well here, too). Add thyme, bring to a boil, then add the meatballs.
6. Coat meatballs in sauce, reduce heat and simmer, covered, for 1 hour.
7. Allow mixture to simmer, uncovered, for a few minutes to allow sauce to thicken slightly.