1Heat oven to 425°F. Spray 12 regular-size muffin cups with cooking spray. In 8-inch skillet, melt butter over medium-high heat. Cook onion in butter 3 to 5 minutes, stirring frequently, until tender.
2In large bowl, stir Bisquick mix and 1 cup of the cheese. In small bowl, stir egg and milk with fork or whisk until well blended. Make well in center of Bisquick mixture; stir in egg mixture just until dry ingredients are moistened. Stir in onion, chicken and chiles.
3Divide batter among muffin cups, filling each two-thirds full. Sprinkle evenly with remaining 1 cup cheese.
4Bake 18 minutes or until golden. Cool 2 to 3 minutes; remove from pan. Serve warm.