Pour water over rice, and let stand 20 minutes. Drain.
Heat a nonstick skillet over medium heat. Coat pan with cooking spray. Add 1/2 cup onion to pan; cook 5 minutes or until tender, stirring occasionally.
Place bread in a food processor; pulse 10 times or until coarse crumbs measure 1 1/4 cups. Combine rice, cooked onion, breadcrumbs, 1/4 cup cilantro, cumin, 1/4 teaspoon salt, oregano, pork, sirloin, and egg whites, stirring well. Shape mixture into 29 (1-inch) meatballs. Chill for 20 minutes.
Heat a Dutch oven over medium-high heat. Coat pan with cooking spray. Add 10 meatballs, and cook 6 minutes, browning on all sides. Remove meatballs from pan; drain well on paper towels. Wipe drippings from pan with a paper towel. Repeat procedure with cooking spray and remaining meatballs. Return pan to heat. Coat with cooking spray. Add remaining 1 1/2 cups onion, carrot, and garlic to pan; sauté 5 minutes or until vegetables are tender. Add tomato to pan, stirring to combine. Stir in broth; bring mixture to a boil. Add meatballs and mint to pan. Reduce heat, and simmer 35 minutes. Add zucchini, remaining 1/2 teaspoon salt, and pepper to pan. Cook 10 minutes. Garnish with remaining 1/4 cup cilantro.
Note: Chilling the meatballs for 20 minutes helps them hold their shape when cooked. Albóndigas (ahl-bon-dee-gas) is Spanish for "meatballs".
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