Wrap tortillas in aluminum foil; bake at 350°F for 15 minutes.
While tortillas bake, heat oil in a large skillet over medium heat. Add onion, and sauté 5 minutes or until tender. Add garlic and cumin, and saute 30 seconds longer. Add spinach, and cook, tossing frequently, just until spinach wilts, about 2 minutes. Combine turkey, spinach mixture, and 1 cup of cheese in a large bowl.
Spread ¼ cup salsa in the bottom of two 11- x 7-inch baking dishes.
Spoon turkey mixture evenly down centers of tortillas. Roll up tortillas; place, seam sides down, in the two baking dishes. Top evenly with remaining salsa and cheese.
Bake for 20 to 25 minutes or until heated through. Let stand 5 minutes. Top with sour cream or Greek yogurt and cilantro, if desired.
To freeze enchiladas, line one 11- x 7-inch baking dish with heavy-duty foil; spray with cooking spray. Make enchiladas as directed through step 4. Cover enchiladas with a layer of foil. Freeze overnight or until enchiladas are firm. Use foil to lift enchiladas out of baking dish. Ensure that foil is sealed tightly, adding an additional layer to cover enchiladas entirely, if needed. When ready to bake, heat oven to 350°F. Place enchiladas back into 11- x 7-inch baking dish. Bake enchiladas, covered with foil sprayed with cooking spray, 40 minutes. Remove foil, and bake about 10 to 15 minutes longer or until enchiladas are heated through and cheese is melted and bubbly. Let stand 5 minutes.