1. Preheat oven to 425°.
2. Heat a large cast-iron skillet over medium-high heat. Coat pan lightly with cooking spray. Add onion to pan; sauté 5 minutes, stirring frequently. Reduce heat to low; cook 20 minutes or until golden brown, stirring occasionally. Cool slightly.
3. 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. Sprinkle with remaining 1 tablespoon chives.