This was adapted from an Epicurious recipe. Using the meat from a rotisserie chicken makes it easier. I make the Tomatillo Cilantro sauce in advance. You could also try using the one from Trader Joe's.
Preheat oven to 450°F. Mix tomatillos, chicken broth, and garlic cloves in a large saucepan. Cover and bring mixture to boil. Reduce heat; simmer gently until tomatillos are soft, about 10 minutes. Transfer hot mixture to processor. Add sliced green onions, chopped cilantro, and sliced chile; blend the mixture to a coarse puree. Season sauce to taste with salt and pepper. You can make this a day or two in advance.
Overlap 6 tortillas in 13x9x2-inch oval or rectangular baking dish. I used scissors to cut some in half for the sides. Top tortillas with half of the chicken strips and half of the mozzarella. Pour 2 cups tomatillo sauce evenly over. Top with remaining tortillas, chicken strips, and mozzarella. Dollop sour cream on top and smooth out partially. Pour 1 1/2 cups tomatillo sauce over, Sprinkle with salt and pepper. Bake until bubbling, about 25 minutes. Cool enchiladas 10 minutes. Serve with remaining tomatillo sauce.
For a vegetarian version, I have also made it without the chicken and added two to four cups of shredded cheddar.