Preheat oven to 400 degrees F. On a rimmed baking sheet, roast the tomatillos, onion, garlic and jalapeños for 15-20 minutes, until the tomatillos are bubbling and juicy. Transfer the roasted vegetables and any juices on the bottom of the tray to a food processor. Add the cumin, salt, and cilantro and pulse mixture until well combined but still chunky.
In a large bowl combine and shredded chicken with 1/3 of the fresh salsa verde, sour cream, and 2 cups of the cheese. Mix well, then salt and pepper to taste.
Change the temperature of the oven to 350 degrees F and begin assembling the dish. Take a large 9x13-inch baking dish and smear the bottom with some of the reserved salsa verde. Put a scoop of the shredded chicken mixture on top of each tortilla. Fold the tortillas over the filling and roll like cigars to close. Place the tortillas in the baking dish; you may have to squeeze them in.
Pour the rest of the salsa verde over the enchiladas and top with the remaining shredded cheese. Bake uncovered for 20-30 minutes until bubbly. Garnish with cilantro, avocado and tomato. Top with a scoop of sour cream!
You can make the salsa verde several days in advance of preparing the chicken enchiladas, and keep them in an airtight container in the fridge.