1. Mash 4 of the garlic cloves to a paste with 1/2 teaspoon of the salt in a small bowl. Combine masa harina with water in a large bowl. Add pork, beef, egg, garlic paste, remaining 1 teaspoon salt, and pepper; mix well. Shape mixture into 1-inch balls.
2. In blender, puree tomatoes with remaining 2 garlic cloves until smooth, then press mixture through a sieve into a small bowl. Puree the chiles and 1 cup water in blender and strain into another small bowl.
3. Heat oil in large Dutch oven over medium heat. Add onion and cook, stirring, 2 minutes. Add tomato mixture, cover, and cook 5 minutes. Stir in bouillon and cumin and cook 2 minutes. Gradually add chile puree, tasting for spiciness after each addition.
4. Add meatballs and bring to a boil. Cover and cook 3 minutes. Reduce heat and simmer 30 minutes, turning meatballs occasionally. Cool. Cover and refrigerate overnight. Reheat over medium heat, stirring gently, about 30 minutes or until hot. Transfer to a serving bowl and garnish with parsley or cilantro, if desired. Makes 7 dozen meatballs.