For sauce: Cook spinach according to package instructions.
Set aside. Melt butter in heavy medium skillet over medium heat. Add flour and stir mixture 2 minutes; do not brown.
Gradually whisk in whipping cream and milk.
Simmer until thickened, about 5 minutes.
Stir in spinach, cilantro, green onion, chilies, cumin, coriander and red pepper.
Puree in batches in processor until almost smooth.
Season with salt and pepper.
(Can be prepared 1 day ahead.
Cover and refrigerate. Bring to room temperature before using.) For enchiladas: Heat oil in heavy small skillet over medium-high heat. Using tongs, briefly dip each tortilla in oil to soften, about 15 seconds per side. Transfer to paper towels and drain.
Combine cheeses in large bowl; set aside 1 1/2 cups for topping.
Combine onion and cilantro in small bowl.
Place 1/4 cup cheese mixture in center of 1 tortilla.
Spoon 2 teaspoons onion mixture over.
Roll up tortilla.
Pla ce seam side down in large glass baking dish.
Repeat with remaining tortillas, cheese and onion, using 1/4 cup cheese for each.
(Can be made 1 day ahead.
Cover and chill.) Preheat oven to 375°F.
Stir sour cream into sauce; pour over enchiladas.
Sprinkle with reserved 1 1/2 cups cheese. Bake until cheese melts and enchiladas are heated through, about 25 minutes.