Preheat oven to 425 degrees.  Heat a cast iron skillet over medium-high heat. 

Melt 1 tbsp of butter or margarine in skillet.  Add onion to pan and sauté 5 minutes, stirring frequently.

Add sugar and stir to combine.  Reduce heat to low.  Cook onions for 30-45 minutes, stirring occasionally.  Be sure to spread onions out on pan as much as possible while cooking.  Continue to cook until deeply golden brown.  Set aside to cool slightly.

Combine half the cheese, caramelized onion, 1 tbsp chives, and the remaining ingredients in a medium bowl. Transfer mixture to a 1 quart baking dish. Top with reserved Gruyere.  Bake for 20 minutes or until bubbly and browned. Sprinkle with remaining chives.  Serve warm.

To make ahead, reserve chive topping, cover and refrigerate. Bake an additional 5-10 minutes. Top with chives and serve.