Saute the Garlic - In a medium saucepan, add olive oil. Heat over medium heat. Once hot, add the garlic and saute for 30 seconds.
Saute the Chilis - Add the chipotle chilis, then stir and break them into smaller pieces. Cook for 1 minute.
Add the Seasonings - Stir in the chicken stock, crushed tomatoes, salt, cumin, black pepper, chili powder, and chipotle chili powder (if using).
Simmer the Sauce - Bring the sauce to a boil over medium-high heat, then reduce to a simmer. Cook until thickened, about 5 minutes.
Puree - Transfer to a blender or food processor and puree on medium speed until smooth, about 30 seconds—season with salt and pepper to taste. Adjust with chicken broth if needed to thin the consistency.
Preheat the Oven - Adjust the oven rack to the center position—Preheat to 400ºF (204ºC).
Prepare the Chicken - Place the chicken between two plastic sheets or in a large plastic bag. Flatten to an even thickness, about ½-inch thick. Repeat with the remaining pieces. Alternatively, cut in half horizontally for pieces 1-inch thick or larger to make cutlets.
Season the Meat - In a small bowl, combine salt, 1 teaspoon of cumin, paprika, garlic powder, and black pepper. Reserve 1 teaspoon of the mixture. Evenly season each side of chicken with the mixture.
Cook the Chicken - Heat a saute pan over medium heat. Add olive oil. Once hot and shimmering, add the chicken. Cook until lightly golden brown, about for 5 to 7 minutes. Flip and cook until no longer pink and the internal temperature reaches 160 to 165ºF (71 to 74ºC), about 5 to 7 minutes. Transfer to a plate to cool.
Shred into Pieces - Use your fingers or two forks to shred the chicken into smaller pieces. Alternatively, chop them with a knife. Add the reserved seasoning mix and combine until evenly coated.
Warm the Tortillas - Place the tortillas in a pan over medium-high heat to lightly toast and make them pliable. Alternatively, warm them directly over a gas flame for 10 to 15 seconds on each side.
Assemble the Enchiladas - Lightly coat a 13 by 9-inch baking dish with cooking spray or oil. Evenly spread ½ cup of the enchilada sauce on the bottom. Pour ½ cup of enchilada sauce into a shallow bowl large enough to fit the tortillas. Lightly dip each side of the tortilla in the sauce.Transfer to a plate and fill the center with about ⅓ cup of chicken. Roll into a cylinder and transfer to the baking dish. Repeat the process until 10 enchiladas are rolled and lined up. Evenly spread the remaining sauce over the top then sprinkle with Monterey jack cheese and cheddar.
Bake the Casserole - Cover the baking dish with foil and bake until the cheese is melted and bubbly, 15 to 20 minutes.
To Serve - Garnish with green onions and cilantro. Serve while still hot.
Topping Suggestions: Diced avocado, diced tomatoes, sour cream, or jalapenos. Sprinkle on soft cheese like cotija or harder cheese like Parmesan just before serving.
Using Pre-Cooked Chicken: 4 cups of shredded roasted or boiled chicken can be used instead of the sauteed chicken.
Using Canned Sauce: Use 2 cups of canned enchilada sauce.
Make it Less Spicy: Reduce or omit dried chili pepper or chipotle pepper. Reduce canned chipotle chilis.
Make it Gluten-Free: Use corn tortillas or cassava tortillas instead of flour tortillas.
Make-Ahead: Assemble the enchiladas, cover, and refrigerate for up to 2 days. Freeze for up to 1 month. Thaw before using.
Storing: Cover and store in the refrigerator for up to 5 days.
Reheat: Cover and warm in a 325ºF (163ºC) oven until hot. Or cover and individually microwave portions in 15-second intervals on high.