Wrap tortillas in aluminum foil and heat in oven at 275F until fillings and sauce are prepared.
Black Bean Filling:
Heat oil in large skillet and add garlic. Grate onion into skillet with a hand grater. Add jalapeno and cook for 1 or 2 minutes. Add beans and mash with fork. Stir in cumin and tomato paste and season with salt as desired.
Warm Salsa Verde:
Place tomatillos in a food processor and pulse to a coarsely ground paste.
Heat garlic in oil over medium heat in a large saucepan. When garlic begins to sizzle, grate onion with a hand grater directly into pan. Add jalapeno. Heat onion and jalapeno through for 1 or 2 minutes and add ground tomatillos. Simmer tomatillos with onion for 5 minutes. Add stock and seasonings.
Halve avocado with skin on by cutting in and down to the pit all the way around avocado. Separate avocado halves and scoop out pit with a large spoon. Scoop flesh out of skins and into pot. Mash with fork. Stir avocado into sauce to thicken it. Return sauce to a gentle boil. Reduce heat to warm until enchiladas are assembled.
Scoop some sauce onto bottom of a casserole dish or shallow serving platter. To make enchilada, place 1 or 2 scoops or filing down center of a tortilla and roll. Place filled tortillas seam side down into sauce on platter or casserole. Line up tortillas one next to another and top with remaining sauce and grated cheeses. Melt cheeses under broiler and serve immediately with Warm Salsa Verde.
* Percent Daily Values are based on a 2,000 calorie diet. Your daily value may be higher or lower depending on your calorie needs.