How to Make It
Toast pepitas in a medium-size cast-iron skillet over low, shaking pan often, until fragrant and toasted, 12 to 15 minutes. Coarsely chop 2 ounces pepitas; set aside. Let pepitas cool 10 minutes. Place remaining 6 ounces toasted pepitas in a food processor. Add tomatillos, cooked rice, breadcrumbs, egg whites, chopped mint, and 2 teaspoons salt; process until smooth.
Transfer tomatillo mixture to a large bowl. Using wet hands, shape mixture into 20 to 22 (1-inch) balls. Set aside on a plate.
Combine oil and onion in an 8-quart stockpot over medium-low. Increase heat to high, and cook until onions are translucent, 4 to 5 minutes. Add tomato paste, cook, stirring often, until slightly darkened, about 2 minutes. Add stock, tomatoes, potatoes, and bay leaf. Bring to a simmer.
When soup is steaming, add meatballs and zucchini. Reduce heat to low, and simmer until meatballs are cooked through, 15 to 20 minutes, adjusting heat as needed to maintain a low simmer. Stir in pepper and remaining 1 tablespoon salt. Carefully ladle soup into shallow bowls (about 4 meatballs per serving). Garnish with crème fraîche or sour cream, mint leaves, and sliced chile, as desired. Serve with lime wedges.