Swiss Chard and Sweet Potato Gratin

Once again, a recipe from the "I should not be cooking while I have the flu" collection.  I don't understand it, I had a giant batch of carrot soup made, plenty of leftovers in the freezer, and no appetite, so why did I need to try new recipes?  In this case, I was worried the swiss chard was going to go bad.  It's such a good-for-you vegetable too (as are sweet potatoes!) so it seemed a shame not to make this dish.

Unfortunately, I was sick and didn't have the patience to let the sauce thicken properly.  Nor did I have the patience to properly drain the chard, so it ended up slightly liquidy to say the least.  Next time, I'd mix the cheese in with the sauce to help ensure it thickened properly and to simplify the layering.  Directions below are made to include this change.

Swiss Chard and Sweet Potato Gratin (from Smitten Kitchen)

Ingredients

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 140g) coarsely grated Gruyére cheese

Prep greens: Cook onion in 2 tablespoons butter in a wide 8-quart heavy pot over moderately low heat, 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 moderately 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 cream 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 warm cream/milk and boil, whisking, one minute. Slowly add in cheese, whisking the entire time.  Continue heating and stirring until sauce is thick.  Season sauce with salt and pepper and fresh or dried herbs..

Assemble gratin: Preheat oven to 400°F. Butter deep 9×13 baking dish. Spread half of sweet potatoes in the prepared baking dish. Top with half of green mixture.  Repeat.  Pour cheese sauce mixture over top.

Bake gratin for about 1 hour until golden and bubbly, and most of the liquid is absorbed. Let stand 10 minutes before serving.