2 1/2 pounds bone-in chicken thighs or legs
1 pound beef bones, or a cut of beef with a lot of bone in it (like short ribs), optional
1 medium onion, quartered (leave the skin on)
1 head garlic, halved across the equator (leave the skin on)
1/4 cup vegetable oil, or more as needed
6 corn tortillas
Salt
2 tablespoons canned chipotle chilies in adobo, or to taste
1/2 cup chopped fresh cilantro
2 avocados, pitted, peeled and cubed
4 to 8 ounces plain melting cheese, like mozzarella (not fresh), Oaxaca or Jack, shredded or cubed
Lime wedges for serving, optional.