Preheat oven to 350 F. Dice onion (and any other preferred vegetables such as bell peppers) and sauté in a medium pan over medium heat. Cook until onions are translucent. Add black beans and spinach. Cook until beans are heated and spinach is wilted.

Spread a thin layer of tomatillo sauce on the bottom of the pan. 

Fill corn tortillas with black bean and vegetable mixture, approximately one half cup. Roll to enclose, and place in a casserole dish, seam side down. **Note- it is important to do this with WARM tortillas. Otherwise they will break as you try to fill and roll the enchiladas. 

Top with additional tomatillo salsa and shredded mozzarella cheese. Bake at 350 degrees until the cheese is melted, approximatly 10 minuets. Garnish with sour cream.

Tip: 4 oz. shredded part skim mozzarella cheese is 1 Cup.