Preheat the Oven - Adjust the oven rack to the center position—Preheat to 400ºF (204ºC).
Flatten the Chicken - Thoroughly dry the chicken breasts with a paper towel. Cut each piece horizontally lengthwise to make two thinner cutlets. Place one of the pieces in a large plastic bag and use a meat tenderizer to flatten it to an even ½-inch thickness. Repeat with the remaining pieces. Season both sides of the chicken breast with salt and pepper.
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 4 to 6 minutes. Flip and cook until no longer pink and the internal temperature reaches 160 to 165ºF (71 to 74ºC) on a meat thermometer, about 3 to 5 minutes. Transfer to a plate to rest for 10 minutes.
Shred into Pieces - Use your fingers or two forks to shred the chicken into smaller pieces. Alternatively, chop them with a knife.
Mix the Filling - In a large bowl, mix the shredded chicken, sour cream, fire-roasted chilies, ¼ cup Pepper Jack cheese, ¼ cup cheddar cheese, green onions, salt, pepper, cumin, and garlic powder. Season with more salt and pepper to taste. Set aside.
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. Transfer to a plate and cover with a kitchen towel to keep them warm and pliable.
Assemble the Enchiladas - Lightly coat a 13 x 9-inch baking dish with cooking spray or oil. Evenly spread ½ cup of the enchilada sauce on the bottom.On a plate, fill the center of a tortilla with about ⅓ cup of filling mixture. Roll into a cylinder and transfer to the baking dish. Repeat the process until ten enchiladas are rolled and lined up. Cover with the remaining sauce, and add Pepper Jack and cheddar cheese on top.
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 cilantro. Serve hot.
Notes
Topping Suggestions: Diced avocado, tomatoes, red onion, sour cream, or jalapenos. Sprinkle on a soft cheese like cotija or a harder cheese like Parmesan before serving.
Using Pre-Cooked Chicken: 3 cups of shredded chicken can be used instead of the pan-seared chicken.
Make it Spicier: Add chipotle powder, chili powder, fresh diced chiles, or spicy fire-roasted chiles to the chicken filling.
Make it Gluten-Free: Use corn or cassava tortillas instead of flour tortillas.
Make-Ahead: Assemble the enchiladas, cover them, 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. Alternatively, cover and individually microwave portions in 15-second intervals on high until hot.