Add chicken, two cups of water, onion, 1 teaspoon of salt, 1 garlic clove to the Instant Pot insert.
Cook on high for 10 minutes. (If you are using the stove do the same. Add the ingredients to a pot but let the chicken cook for about 40 minutes until it is well cooked, and easy to shred.) Quick release is once done.
Meanwhile, in a saucepan combine 2 cups of water and the tomatillos and boil for about 5 - 10 minutes. Remove from the heat.
While the tomatillos are boiling AND once the chicken is ready, remove it from the Instant Pot and shred it by using 2 forks and pulling it apart. Set aside.
In a skillet., add 1 tablespoon of oil, and heat the tortillas one at a time for just a couple of minutes - letting them get hot and adequately covered in oil - but don't cook them to the point where they start to harden or they won't roll. You want them pliable. (This will help the tortillas remain in intact and not break while you are rolling the enchiladas.)
In a blender, blend the cooked tomatillos, the water used to boil them, chiles, cilantro, onion, and the remaining two garlic cloves.
Transfer the blended mixture to a saucepan, add 1 tablespoon of oil, salt and pepper to taste. If you would like your sauce to be thinner, you can add the extra water. Bring to a simmer and then remove from the heat.
Take your baking dish and add a layer of sauce - about 1/2 cup or just enough to cover the bottom. Set aside.
Preheat the oven to 350ºF.
Dip a heated tortilla in the sauce and then place it on a plate and add about 1 tablespoon of shredded chicken done the center.
Drizzle about 1 tablespoon of sauce over the chicken.
Roll the tortilla and place it in the baking dish.
Repeat with the remaining tortillas and place them all next to each other until you fill your dish.
Once they are all placed, pour some of the sauce over the top - about 1 cup or to your liking depending on if you like it saucy or with just a little sauce.
Add the sour cream and add the cheese in an even layer on top.
Bake 15 to 20 minutes, or until the cheese is completely melted and bubbling.
Remove from the oven and serve warm.