Preheat oven to 375°F. Lightly spray a 9×13 pan with cooking spray. Spread 1 cup salsa verde on bottom of pan.
In small frying pan, sauté onions with small amount of olive oil until softened, about 5 minutes.
In large bowl, mix shredded chicken with ⅔ cup salsa verde, 1 ½ cups cheese, scant ¼ cup cilantro, sautéed onions, and salt and pepper to taste.
Wrap tortillas in two paper towels and microwave for 30 to 45 seconds so that they are soft and easier to work with.
Spoon about ¼ cup of the chicken mixture into a tortilla. Roll up tightly and place seam side down in the baking dish. Repeat until you are out of filling. Drizzle enchiladas with remaining salsa verde (about ⅓ cup) and sprinkle with remaining cheese.
Spray a large sheet of foil with cooking spray and use that to cover the baking dish. Bake for 25 minutes, remove foil and bake for five more minutes.
Sprinkle with remaining cilantro, let stand for a few minutes and serve, topped with sour cream, if desired. One serving is 2 enchiladas.
Vegetarian: Instead of chicken, fill the enchiladas with fajita-style vegetables (grilled peppers and onions), and a can of black beans, rinsed and drained.