Preheat the oven to 400 F.
In a small bowl, mix together the chicken, 1/2 cup salsa verde, 1 cup of the cheese, 1/4 cup cilantro and salt and pepper to taste.
Spread half of the rest of the salsa in the bottom of a 7x11 baking pan. (This fit 8 enchiladas perfectly. You could probably also squeeze them into a 9x9 but this odd sized pan I have is just perfect for stuff like this and I use it more and more now that there are just two of us.)
Heat the tortillas. I have a tortilla warmer that goes in the microwave. You could also wrap tortillas in paper towels and microwave. Or heat in a cast iron skillet until softened. They just roll better once they're warmed up a bit.
Spoon about 1/4 cup of the chicken mixture onto a tortilla and roll it up. Place it seam side down in the baking dish. Repeat until you're done.
Top the enchiladas with the rest of the salsa and grated Jack cheese. Take a piece of aluminum foil and rub a bit of oil across it to prevent it from sticking to the enchiladas, then cover the enchiladas with the foil, oiled side down. Bake for 30 minutes.
Take the dish out of the oven and take off the foil. Top the enchiladas with the onion slices and the rest of the cilantro. Let sit a few minutes while you fix everything else, then dish up.
Serve with the rest of the salsa verde and sour cream to add at the table.