Warm a nonstick skillet over medium heat. Cook the tortilla one at a time on both sides. When the tortilla has opaque white spots and some brown spots, transfer to a clean dish and set aside. Repeat with the remaining tortillas.
In the same skillet over low heat add ½ cup of the sauce and mix in the water to thin out. Warm the sauce for about 2 minutes and then off the heat.
To assemble, coat both sides of a tortilla in the sauce. Let the excess drip off. Transfer the coated tortilla to a clean plate and add a few tablespoons of the spinach and cheese filling down the center. Tightly roll the tortilla. Transfer to the baking dish.
Repeat with the remaining tortillas.
Pour over the remaining enchilada sauce.
Sprinkle with the remaining ¼ cup of cheese.
Cover with foil and bake for 20 minutes until the cheese has melted.
*You can also use frozen spinach in the same amount. Just make sure to thaw and squeeze out the liquid.