Preheat the oven to 425. Place the onion quarters, tomatillos and unpeeled garlic on a baking sheet and drizzle with olive oil. Roast the vegetables until they begin to blacken and blister, about 35-40 minutes. Squeeze the garlic from the peels and discard the peels. Place the vegetables in a blender along with the beef broth, chipotles and the 1/2 teaspoon salt. Process until smooth. Pour the sauce into a small saucepan, bring to a simmer, reduce the heat to low and simmer for 10 minutes, stirring occasionally. Add more salt to taste.
In a large bowl, combine the ground beef, chorizo, breadcrumbs, milk, eggs, oregano, salt, pepper and cilantro. Using your hands, knead the mixture until thoroughly combined. Form the meat mixture into 1-inch balls, flatten them and place a piece of hard-boiled egg, and close the meat around it to form back into a ball. Repeat for all meatballs. Makes about 30 meatballs.
Heat some vegetable oil in a large skillet over medium-high heat and fry the meatballs on all sides until brown. Transfer the meatballs to paper towels to drain the grease. Pour the sauce into the skillet and nestle the meatballs in the sauce. Bring to a simmer, reduce the heat to low, cover and simmer for 20 minutes, stirring occasionally to prevent burning. Add a little more beef broth as needed to think the sauce.Serve with your choice of sides (e.g. rice, crusty bread, warm corn tortillas, a salad).