Pat the chicken thighs dry with a paper towel, then salt and pepper and add some chili powder if you like it spicy.
Cut the onion in half at the equator and then half again as if you were making extra thick onion rings. Leave the tomatillos and garlic in their papery wrappers and the hatch chile peppers whole.
Grill the ingredients
Place the head of garlic off on the edge of the flame and grill while the other ingredients are grilling making sure to turn it regularly.
Grill the hatch chile peppers until they have a nice char on their skin then move off to the side away from the flame.
Grill the onions until they have dark grill marks on each side and move them over to the side with the peppers.
Finally, grill the chicken thighs for 6-8 minutes per side allowing them to develop grill marks. Don't worry too much about the internal temperature of the chicken since it will be cooked again in the enchiladas.
Place the hot roasted hatch chile peppers in a paper or plastic bag or a bowl covered with plastic wrap. Let the peppers "sweat" (the steam will help loosen the skin) for 15 minutes while preparing the remaining ingredients for the sauce.
Make the tomatillo sauce
Rough chop two-thirds of the onion and add to the pot, stirring occasionally until they turn translucent.
Cut the root end of the garlic head off and gently squeeze the roasted cloves into the pot, turning it down to make sure the garlic doesn't burn.
Remove the papery skins off the tomatillos and remove the stem, then roughly chop them and add them to the pot.
Gently peel off the skin of the hatch chile peppers, remove the stem, and rough chop the add about a third of them to the pot.
Give everything a thorough mix then turn it back up to medium-high. Once it's bubbling, add the chicken broth and use an immersion blender to get the sauce to a smooth texture. You can pour the contents into a regular blender or food processor if that's what you have available.
Finally, turn it down to medium-low to simmer for at least an hour, preferably two, until reduces by 20% or so, intensifying the flavor.
Assemble the enchiladas
Cut the chicken into half-inch cubes and set aside. Roughly chop the remaining hatch chile peppers and onions and mix them with the chicken.
Spread about a third of a cup of the sauce on the bottom of an 8 by 13-inch casserole or baking dish. Lay out a tortilla and place the chicken mixture in the middle. Roll the enchilada up and place them seam side down in the dish.
You should have room for six well-stuffed enchiladas, and once they are all in cover them with the rest of the sauce. Sprinkle shreds of the Oaxaca cheese over the top, adding as much as you want depending on your preference.
Bake
Place the casserole in a preheated 350-degree Fahrenheit oven for 12-15 minutes, or until the cheese and sauce have started to merge and are bubbling.
Let the enchiladas cool for at least 5 minutes before plating. Top with sliced avocado, guacamole, sour cream, cilantro, and/or tomatillo green chile sauce.
Notes
1. You can grill the ingredients in advance and cook down the sauce and refrigerate everything for a few days. The assembly of the enchiladas and oven time is minimal which makes for a perfect weeknight dinner.2. If you have store-bought tortillas you may want to steam them in the microwave first to make them more pliable. Wrapping them in a moist paper towel and cook on high for 30 seconds.3. Grilling the chicken, onion, peppers, and garlic is an optional step, but it brings so much to the flavor profile of the final dish that we highly recommend putting in the effort.4. These enchiladas can be reheated in a microwave or in the oven. For microwave reheating, place on a microwave-safe plate and heat at 30-second intervals until thoroughly warmed. To reheat in the oven, place in an oven-safe dish and reheat at 350 degrees Fahrenheit for about 10 minutes or until thoroughly warmed.5. These are perfect for freezing. Make a double batch so you can eat one and freeze one for a quick weeknight dinner. For easy baking freeze these in a disposable pan and cover tightly with foil then place in a freezer-safe air-tight bag. Defrost leftovers in the refrigerator and then bake according to recipe card directions.6. Nutritional information is only an estimate. The accuracy of the nutritional information for any recipe on this site is not guaranteed. The nutritional value is only for the enchiladas and does not include the nutritional value for any garnishes such as sour cream or avocado. The nutritional value does not include any additional toppings.7. Be sure to check out the Key Ingredients along with the Recipe Tips and Notes sections found in the post, for more tips, options, substitutions, and variations for this recipe! You will also find step-by-step photos there.