Adjust oven rack to center position and heat oven to 400 degrees. Mix beans with spinach, chilies, 3/4 cup salsa verde, half the cheese, the cilantro, and salt and pepper to taste. Spray a 13-by 9-inch and a second small baking dish large enough to hold an additional 4 enchiladas.
Working one at a time, spoon about 1/4 cup of the filling into a tortilla. Roll into a cylinder and place, seam-side down, in prepared baking dish. Repeat with remaining tortillas and filling. Drizzle with enchilada sauce and sprinkle with remaining cheese. Cover with a cooking spray-coated sheet of foil and bake until heated through, about 30 minutes.
Turn oven to broil, remove foil, sprinkle with onion slices and continue to cook until cheese is bubbly, let stand for a few minutes, sprinkle with cilantro, and serve with sour cream and remaining salsa verde.