Heat Oven To 350 Degrees F. Mix Sour Cream And Salsa Until Well Blended. Cook And Stir Onions And Garlic In Hot Oil In Large Skillet On Medium Heat 4 To 5 Min. Or Until Vegetables Are Crisp-Tender. Stir In Chicken, 1 Cup Sour Cream Mixture, Chiles And 1/2 Cup Cheese. Spoon About 1/3 Cup Chicken Mixture Down Center Of Each Tortilla; Roll Up. Place, Seam-Sides Down, In 13X9-Inch Baking Dish Sprayed With Cooking Spray. Top With Remaining Tomatillo Sauce And Cheese. Cover. Bake 15 To 20 Min. Or Until Heated Through.