Preheat your oven to 425 degrees Fahrenheit.
Place the quartered onion, tomatillos, and unpeeled garlic on a baking sheet. Drizzle them with a bit of olive oil.
Roast the vegetables for about 35 to 40 minutes until they begin to blacken and blister.
Once roasted, take the baking sheet out and let the vegetables cool slightly. Squeeze out the roasted garlic.
Transfer the roasted tomatillos, onion, and garlic to a blender. Add the beef broth, chipotles, and 1/2 teaspoon of salt. Blend until smooth.
Pour the sauce into a small saucepan and bring it to a simmer over medium heat.
While the sauce simmers, prepare the meatballs by combining the ground beef, chorizo, breadcrumbs, milk, eggs, oregano, salt, black pepper, and chopped cilantro in a large bowl.
Using your hands, knead the mixture until thoroughly combined.
Form the meat mixture into 1-inch balls, flatten each ball, place a piece of hard-boiled egg in the center, and mold the meat around it back into a ball.
Heat some vegetable oil in a large skillet over medium-high heat. Fry the meatballs until brown on all sides.
Once browned, transfer the meatballs to paper towels to drain excess grease.
Pour the prepared sauce into the skillet and nestle the meatballs into the sauce. Bring to a simmer.
Reduce heat to low, cover, and let it simmer for about 20 minutes.
Serve hot, alongside your choice of sides like rice or warm corn tortillas.