Optional toppings: chopped tomatoes, sliced ripe olives and sliced green onions
Preheat oven to 350°. Mix first four ingredients until smooth.
In a small saucepan, heat butter over medium-high heat; saute onion until tender. Add garlic; cook and stir 1 minute. Stir in chilies and flour mixture. Bring to a boil; cook and stir until thickened, 1-2 minutes. Remove from heat; stir in 1/2 cup cheese and sour cream until cheese is melted.
In a bowl, mix chicken with 3/4 cup sauce. Place 1/2 cup mixture on each tortilla; roll up and place in a greased 11x7-in. baking dish, seam side down. Top with remaining sauce.
Bake, uncovered, 20 minutes. Sprinkle with remaining cheese; bake, uncovered, until cheese is melted, 5-10 minutes. If desired, serve with toppings.
Health Tip: Make a healthier version of this casserole: Use just ½ cup cheese, reduced-fat sour cream, white meat chicken and whole wheat tortillas.