Heat oven to 350°F. Spray 13x9-inch (3-quart) baking dish with cooking spray. In medium bowl, mix sour cream, yogurt, soup and chiles.
Spoon about 3 tablespoons sour cream mixture down center of each tortilla. Reserve 1/4 cup of the cheese; sprinkle tortillas with remaining cheese, chicken and onions. Roll up; place in baking dish. Spoon remaining sour cream mixture over tortillas. Cover with foil.
Bake 25 to 30 minutes or until hot and bubbly. Sprinkle with reserved 1/4 cup cheese. Bake uncovered 5 minutes longer or until cheese is melted. Garnish with shredded lettuce and chopped tomatoes.