Preheat the oven to 350°F. Lightly grease a 13×9-inch baking dish.
In a small bowl, stir together 1/4 cup of the salsa and both 10-ounce cans of enchilada sauce. Set this sauce mixture aside.
Place the cream cheese in a medium microwave-safe bowl and microwave on HIGH for 1 minute or until very soft.
Make sure the thawed frozen spinach is well squeezed to remove excess liquid. Add the spinach, the 2½ cups shredded or chopped chicken, and the remaining salsa from the jar (the jar minus the 1/4 cup reserved) to the softened cream cheese. Stir until evenly blended.
Spoon a heaping 1/3 cup of the chicken-spinach mixture down the center of each 8-inch flour tortilla. Roll each tortilla up and place seam-side down in the prepared 13×9-inch baking dish.
Pour the reserved enchilada sauce mixture evenly over the rolled tortillas. Sprinkle the 8-ounce package of shredded Mexican cheese blend evenly over the top.
Bake for 30 minutes, or until the sauce is bubbly and the cheese is melted. Let the enchiladas stand 5 minutes before serving.
Serve with desired extra toppings (shredded lettuce, guacamole, chopped cilantro, chopped red onion, halved grape tomatoes, as desired).