Preheat oven to 375°F.
In a small bowl, combine the 15 oz rinsed and drained black beans and the 4 oz mild green chiles; set the mixture aside.
Pour the 2 cups enchilada sauce into a shallow bowl or pie plate. Spread some sauce from the bowl over the bottom of a 9×13-inch baking dish.
Working one tortilla at a time, dip a 6-inch tortilla into the sauce to lightly coat both sides, then transfer it to a plate.
Place a spoonful of the bean mixture in the top center of the sauced tortilla (use enough to fill but still allow rolling). Top the beans with some of the shredded Monterey Jack or Mexican blend cheese. Roll the tortilla tightly and place it seam-side down in the prepared baking dish. Repeat steps 4–5 with the remaining tortillas, arranging them close together in the pan.
Pour any remaining enchilada sauce evenly over the rolled enchiladas, then sprinkle the rest of the shredded cheese over the top.
Bake uncovered in the preheated oven until the cheese is melted and bubbly, about 20 minutes.
Remove the pan from the oven and let the enchiladas rest for 5 minutes.
Garnish with the 1/3 cup chopped cilantro, the sliced green onions, sliced avocado, and the 1/3 cup crumbled queso fresco. Serve warm.