- 1/4 cup butter
- 1 small onion, finely chopped
- 3 pounds Swiss chard, leaves and stems each cut into 1-inch pieces
- Pinch of freshly grated nutmeg
- 2 cups whole milk
- 2 garlic cloves, minced
- 2 tablespoons flour
- 2.5-3 pounds medium red-skinned sweet potatoes (yams), peeled and cut into 1/8-inch thick rounds
- 1 tablespoon minced parsley
- 1 tablespoon minced thyme
- Freshly ground black pepper
- 1 1/4 cups (about 5 ounces) coarsely grated Gruyére cheese
Prep greens: Heat 2 tablespoons butter over medium-low heat in a large heavy pot. Add onion and cook, stirring, until softened. Add chard stems, pinch of nutmeg, and salt and pepper to taste and cook, stirring, until vegetables are tender but not browned, about 8 minutes. Increase heat to medium-high and add chard leaves by large handfuls, stirring, until all greens are wilted. 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 milk and garlic in small saucepan; bring to simmer over medium heat; keep warm. Melt two tablespoons butter in a medium heavy saucepan over medium heat and stir in flour. Whisk one minute, then slowly whisk in warm milk and boil, whisking, one minute. Season sauce with salt and pepper.
Assemble gratin: Preheat oven to 400°F (or bake later). Butter deep 9×13 baking dish. Add the following layers:
- Half of sweet potatoes, salt, pepper, a quarter of the herbs and a 1/4 cup of the cheese.
- Half the greens mixture, salt, pepper, a quarter of the herbs and 1/4 cup of the cheese over it.
- Half the sauce.
- Repeat 1-3, then press the vegetables slightly to ensure that they are as submerged as possible. Sprinkle with the last 1/4 cup of cheese.
Can refrigerate for up to a day at this point. Bake gratin for about 1 hour until golden and bubbly, and most of the liquid is absorbed. Let stand 10 minutes before serving.