Preheat oven to 425 degrees F (220 degrees C). Spray a 3-quart casserole dish with cooking spray.
Combine tomato sauce with chili powder and 1/2 cup water or as desired in a small saucepan over medium heat; bring to a simmer and stir to make a pourable sauce.
Remove from heat and set enchilada sauce aside.
Cook and stir ground turkey, onion, and 1 envelope of taco seasoning mix in a large skillet over medium heat, chopping turkey as you cook, until meat is crumbly and no longer pink, about 10 minutes. Stir in 2/3 cup water and remaining envelope of taco seasoning mix and cook, stirring often, until moisture has evaporated from turkey mixture.
Mix 1 cup of enchilada sauce into turkey mixture and remove skillet from heat.
To layer the casserole, spread 8 tortilla halves into the bottom of the prepared baking dish, overlapping as necessary to fit. Toss sharp Cheddar cheese and Monterey Jack cheese together in a bowl and spread 1/4 of the cheese mixture over the tortillas in an even layer. Top with 1/3 the turkey mixture, 1/3 the lettuce, 1/3 the chopped tomatoes, and about 1/3 cup of the remaining sauce. Repeat layers twice more.
Place 8 tortilla halves on top of the casserole and spread with remaining 1 1/4 cup shredded cheese mixture.
Bake in the preheated oven until cheese on top of casserole is melted and browned, 20 to 30 minutes.
