Kale and Caramelized Onion Gratin

3 tablespoons unsalted butter

4 cups red onion, chopped

2 tablespoons sugar

1 teaspoon fresh thyme leaves

3/4 teaspoon salt

1/2 teaspoon of ground black pepper

2 bunches kale, ] ribs removed, torn into 1 1/2-inch pieces

8 ounces Gruyère cheese, shredded

1/2 cup Parmesan cheese, grated

2 1/2 cups heavy cream

1. Melt 1 tablespoon of the butter in a frying pan 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 frying pan 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 Gruyère 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 Circulon baking pan. Pour the cream over the kale then sprinkle with the remaining Gruyère 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.