Kale and Caramelized Onion Gratin

Ingredients

3 tablespoons unsalted butter
4 cups chopped red onion (about 4 medium)
2 tablespoons sugar
1 teaspoon fresh thyme leaves
3/4 teaspoon salt
1/2 teaspoon ground black pepper
2 bunches kale, about 16 ounces each, ribs removed and torn into 1 1/2-inch pieces
8 ounces Gruyere cheese, shredded
1/2 cup grated Parmesan cheese
2 1/2 cups heavy cream

Directions

1. Melt 1 tablespoon of the butter in a large nonstick skillet over medium heat. Add the onion, sugar, thyme, 1/4 teaspoon of the salt and 1/4 teaspoon of the pepper; cook, stirring occasionally, until golden, about 18-20 minutes. Transfer to a bowl.
2. Preheat the oven to 425°F.
3. Wipe out the skillet and melt 1 tablespoon of the remaining butter over medium heat. Add half of the kale and cook, tossing, until slightly wilted, about 1 1/2 minutes. Transfer kale to the bowl with the onions; repeat with the remaining butter and kale.
4. Add half of the Gruyere cheese, 1/4 cup of the Parmesan cheese, remaining 1/2 teaspoon salt and 1/4 teaspoon pepper to the kale mixture and toss well. Transfer to a nonstick 9 by 13-inch baking pan. Pour the cream over the kale then sprinkle with the remaining Gruyere and Parmesan cheeses.
5. Bake in the center of the oven until the cream has thickened and the cheese is golden brown, about 53-55 minutes. Let stand 10 minutes before serving.