Preparation

• Preheat the oven to 375˚. Butter a 9-by-13-inch baking dish.
• Heat the olive oil and 2 tablespoons butter in a large skillet over medium-low heat.
• Add the onions, 1 teaspoon salt, a few grinds of pepper and the sugar. Stir, then cover and cook, stirring once, for 10 minutes.
• Uncover, increase the heat to medium and cook, stirring for 10 more minutes.
• Add the thyme, sherry and broth. Cook, stirring, until the liquid evaporates and the onions are caramelized, about 15 more minutes.
• Melt the remaining 2 tablespoons butter in a Dutch oven over medium heat. Stir in the flour and cook for 1 minute, then whisk in the heavy cream and milk.
• Add the remaining ½ teaspoon salt and the nutmeg. Bring to a simmer, whisking until smooth. Cook, stirring, until thickened, 2 to 3 minutes. Remove from the heat.
• Stir in all but ½ cup each of the gruyère and Swiss until melted. Stir in the potatoes.
• Layer half of the potatoes in the baking dish, then half of the onions. Repeat with the remaining potatoes and onions.
• Sprinkle with the reserved gruyère and Swiss and the parmesan. Cover with foil, tenting it so it doesn’t touch the cheese; bake until the potatoes are tender, 45 to 55 minutes.
• Uncover and bake until golden brown, 20 to 25 minutes.
• Let sit for 20 minutes before serving. Top with chives.