Adapted from SmittenKitchen
The original recipe called for Swiss Chard, but I just used a frozen bag of spinach, and it was delish.
1/4 cup (1/2 stick or 2 ounces) butter
1 small onion, finely chopped
1 frozen block of spinach (thawed in microwave, extra water squeezed out)
Pinch of freshly grated nutmeg
1.5 cups half and half or whole milk
2 garlic cloves, minced
1.5 tablespoons flour
1 gigantic sweet potato, peeled and cut into 1/8-inch thick rounds
1 tablespoon minced fresh Italian parsley (optional)
1 tablespoon minced fresh thyme (or dried)
Fine sea salt
Freshly ground black pepper
1 1/4 cups (about 5 ounces) coarsely grated Gruyére or Parmesan cheese
Cook onion in 2 tablespoons butter in a wide 8-quart heavy pot over moderately low heat, stirring, until softened. Add spinach, pinch of nutmeg, and salt and pepper to taste and cook, stirring, until vegetables are tender but not browned. Season with salt and pepper then transfer greens to a colander to drain well and press out liquid with back of a large spoon.
Make sauce: Combine half and half or milk and garlic in small saucepan; bring to simmer; keep warm. Melt two tablespoons butter in a medium heavy saucepan over moderate heat and stir in flour. Cook roux, whisking, one minute, then slowly whisk in half and half/milk and boil, whisking, one minute. Season sauce with salt and pepper and allow to thicken.
Assemble gratin: Preheat oven to 400°F. Butter deep 9×13 baking dish. Spread half of sweet potatoes in the prepared baking dish. Sprinkle with salt, pepper, a quarter of the herbs and a 1/4 cup of the cheese. Distribute half of the greens mixture over the cheese, then sprinkle salt, pepper, some of the herbs and 1/4 cup of the cheese over it. Pour half of bechamel sauce over this first layer, then continue with the remaining sweet potatoes, more salt, pepper, herbs and cheese and then the remaining greens, salt, pepper and herbs. Pour the remaining sauce over the top of the gratin, making sure the entire dish is covered by the sauce. Sprinkle with the last 1/4 cup of cheese.
Bake gratin for about 1 hour until golden and bubbly, and most of the liquid is absorbed. Let stand 10 minutes before serving.