How to Make It
Preheat a large cast-iron skillet or griddle. Add the tomatoes and garlic cloves and roast over high heat, turning occasionally, until softened and charred in spots, about 12 minutes; let cool slightly. Peel the garlic and transfer it to a blender along with the tomatoes and chipotles; puree until smooth.
In a large saucepan, heat the olive oil. Add the onion and cook over moderate heat, stirring, until lightly golden, about 5 minutes. Add the chorizo and cook over moderately high heat, stirring to break up the meat, until no trace of pink remains, about 5 minutes. Stir in the tomato puree, stock and bay leaf and bring to a boil. Simmer over low heat until the sauce is thickened, about 1 hour. Discard the bay leaf and season the tinga with salt and pepper.
Preheat the oven to 350° and set a rack over a large rimmed baking sheet. In a large bowl, whisk the egg with the milk. Stir in the tortilla chips, cheese, garlic, oregano, 1 1/2 teaspoons of salt and 1 teaspoon of pepper. Add the ground pork and beef and knead gently to combine.
Form the meat into 1 1/2-inch meatballs and transfer them to the rack. Roast for 25 to 30 minutes, until the meatballs are browned and cooked through.
Add the meatballs to the sauce; toss carefully to coat. Transfer the meatballs to bowls and serve with warm tortillas.