1. Preheat oven to 350°. Sauté onion and garlic in hot oil in a large skillet over medium heat 5 minutes or until tender. Add spinach and green chiles; sauté 1 to 2 minutes or until spinach is wilted. Stir in chicken and next 3 ingredients, and cook, stirring constantly, 5 minutes or until cheeses melt. Add salt and pepper to taste. Spoon about 3/4 cup chicken mixture down center of each tortilla; roll up tortillas.
2. Place rolled tortillas, seam sides down, in a lightly greased 13- x 9-inch baking dish. Lightly coat tortillas with cooking spray.
3. Bake at 350° for 30 to 35 minutes or until golden brown. Top with Tomatillo Salsa.
To Make Ahead: Prepare recipe as directed through Step 2. Cover and chill overnight. Let stand at room temperature 30 minutes. Uncover and proceed as directed in Step 3.