Preheat the oven to 350 degrees F. Melt 4 tablespoons butter in a stoveproof gratin dish over medium-high heat; add 4 pounds Vidalia onions (cut into wedges) and cook 5 minutes. Add a pinch each of nutmeg, cayenne, salt and black pepper, then add 3 tablespoons white wine and simmer 2 minutes. Whisk 1 1/4 cups half-and-half and 3 tablespoons flour; stir into the onions and bring to a boil. Mix 2 tablespoons melted butter, 1 cup panko (Japanese breadcrumbs), 2 tablespoons chopped chives, and salt and pepper. Sprinkle over the onion, cover with parchment paper and bake 30 minutes. Uncover and broil until golden, 5 more minutes.