Heat oven to 350 degrees. In medium-size bowl mix chicken, 1 1/2 cups cheese, and chopped red onion. Place 1/3 cup mixture on bottom half of each tortilla and roll up. Arrange rolled tortillas, seam side down, in 13x9x2-inch baking dish. Set aside. Melt butter in heavy 2-quart saucepan over low heat. Sprinkle in flour, stirring constantly 5 full minutes. Add broth, continuing to stir. Increase heat to medium, stirring about 3 minutes until mixture begins to bubble and thicken. Remove from heat; fold in sour cream and chilies. Pour sauce over rolled tortillas in baking dish, then bake, uncovered 15 minutes. Sprinkle with remaining 1 cup cheese and scallions. Bake 5 minutes longer, until cheese melts.