1/4 cup (1/2 stick or 2 ounces) butter
1 small onion, finely chopped
3 pounds Swiss chard, leaves and stems separated and both cut into 1-inch pieces
Pinch of freshly grated nutmeg
2 cups heavy cream or whole milk
2 garlic cloves, minced
2 tablespoons flour
2 pounds medium red-skinned sweet potatoes (yams), peeled and cut into 1/8-inch thick rounds
1 tablespoon minced fresh Italian parsley
1 tablespoon minced fresh thyme
Fine sea salt
Freshly ground black pepper
1 1/4 cups (about 5 ounces) coarsely grated Gruyére cheese