1 Poach chicken thighs: Put chicken thighs in a medium sized saucepan and just cover with water. Add the halved garlic, 1/4 of an onion, and 2 teaspoons of salt to the water.
Bring to a simmer, reduce the heat to very low to maintain the heat at just below a simmer, and cook, covered, for 10 to 15 minutes, or until the chicken is just barely cooked through. (Here are the step-by-step instructions for How to Poach Chicken.)
Remove chicken thighs to a separate bowl and let cool enough to touch.
2 Cook the tomatillos: While the chicken is cooking, put the tomatillos and serrano chile peppers in a separate sauce pan and cover them with water. Bring to a boil, reduce to a simmer. Simmer until the tomatillos are cooked, and have changed color, but are not mushy, about 5 minutes. Remove from heat.
3 Make the salsa verde: Use a slotted spoon to transfer tomatillos and serrano chiles to a blender. Add 1/4 cup of the tomatillo cooking liquid to the blender, as well as 2 cloves of garlic, a half cup of chopped onion, 1 tablespoon of lime juice, and about 1/2 cup of chopped cilantro. Add one teaspoon of salt.
Purée until blended, 15 to 30 seconds. Taste for heat. If not spicy enough add another chile pepper (it doesn't have to be cooked). Add more salt to taste if necessary.
You should have about 3 cups of salsa.
4 Shred chicken meat, toss with salsa verde: Remove the cooked chicken meat from the bones (if using bone-in thighs).
Shred the meat with a fork or knife. Put the chicken in a bowl and add 1/2 cup of the tomatillo sauce to the chicken. Taste the chicken; if it needs salt, add a little.
5 Soften the tortillas: Heat a tablespoon of oil in a frying pan on medium high heat. Add a tortilla to the pan and let it sizzle for 10 seconds or so. Use a metal spatula to flip it to the other side. Let it sizzle for a few seconds. Place another tortilla on top of this one (to soak up the excess oil) and flip again.
You can keep going, stacking corn tortillas on top of the tortillas that appear to have excess oil. This way the new tortillas absorb some of the excess fat from the other tortillas.
When the tortillas are heated through, remove them to a plate lined with paper towel. Add a more oil to the pan as needed, and continue to heat through and soften all of the tortillas.
6 Roll up tortillas with chicken: Preheat the oven to 250°F. Scoop a spoonful of chicken into the center of the tortilla and roll up the tortilla. Place the rolled tortilla seam side down in a casserole dish and repeat with all of the tortillas.
7 Cover with foil and warm in oven: Cover the rolled tortillas with foil and keep warm in a 250°F for 10 minutes.
8 Heat salsa and smother the rolled tortillas with it: Place the remaining tomatillo salsa in a small saucepan and heat on medium high heat until simmering. Remove warmed rolled tortillas from the oven and smother with the salsa.
Let some of the sauce go between the rolled up tortillas so that it gets to the bottom of the pan.
8 Add sour cream and garnishes: Top with crumbled cotija cheese, chopped red onion, and chopped fresh cilantro.
Thin the sour cream with a tablespoon of water and drizzle all over the top of the enchiladas. (Don't skip the sour cream! It's essential to balance the sharp acidity of the salsa verde.)