Heat a large cast-iron skillet over medium-high heat. Coat pan lightly with cooking spray.
Add onion to pan; saut 5 minutes, stirring frequently. Reduce heat to low; cook 20 minutes or until golden brown, stirring occasionally. Cool slightly.
Reserve 2 tablespoons cheese.
Combine remaining cheese, caramelized onion, 1 tablespoon chives, and the remaining ingredients in a medium bowl.
Transfer the mixture to a 1-quart glass or ceramic baking dish coated lightly with cooking spray.
Sprinkle with reserved 2 tablespoons cheese.
Bake at 425 for 20 minutes or until browned and bubbly.