Preheat oven to 350 degrees. Lightly coat a mufﬁn tin with cooking spray. Place tortilla in the mufﬁn tin, gently press with ﬁngers to form a cup shape. Set aside. Melt butter in large skillet. Add chopped onions to butter and sauté on medium-low heat until soft and translucent. Remove from heat. Do not drain. Cook or grill chicken breasts. Cut into small pieces. Add chicken, sour cream, cream of chicken soup and green chiles to the sautéed onion. Mix until evenly blended. Spoon chicken mixture in the middle of each tortilla cup. Top each cup with shredded cheddar cheese. Bake in preheated oven for 30 to 35 minutes or until bubbling and cheese is melted.