Spinach Gratin

Every year around Thanksgiving and Christmas we get together with all of our friends here in Atlanta for a big holiday potluck dinner. This was the 4th year we've done it, and it's quickly become one of my favorite days of the year. There are 3 couples who have taken over as annual hosts, and they provide the turkeys, mashed potatoes and a few other basics, while all others are responsible for a dish within an assigned category. We have around 20-30 people, or 10-15 couples total, and at least one half of each couple is a darn good cook, so it's always a smorgasbord of really good food and company. It's one of the few occasions where everyone is in the same place at the same time, so we get a little dressed up and break out the silver, crystal and china, and make a long evening out of it, starting with cocktails and apps and ending with dessert (and more cocktails).

While categories are assigned for the potluck portion, they're only limited to appetizer, side or dessert, so it can be a guessing game to make sure all the usual sides are represented with no overlap, but it also gives us the chance to try out new recipes and mix things up. In the past I've brought hashbrown casserole at the request of others, but I have a time thinking of it as a holiday dish, so I've been trying to branch out. Another year I brought a butternut squash gratin, which was good, but I've since decided I'm not the biggest butternut squash fan (a bit too sweet for me), so this year I was faced with a blank slate. After more contemplation than a potluck side dish should allow, I ended up going with Ina Garten's Spinach Gratin which, as is typical of Ina's recipes, did not disappoint in the least. Her recipe, with my slight changes, is as follows.

Ingredients
4 tablespoons (1/2 stick) unsalted butter
4 cups chopped yellow onions (2 large)
¼ cup flour
¼ teaspoon grated nutmeg
1 cup heavy cream
2 cups milk
3 pounds frozen chopped spinach, defrosted (5 (10-ounce) packages)
1 cup freshly grated Parmesan cheese
1 tablespoon kosher salt
½ teaspoon freshly ground black pepper
½ cup grated Gruyere cheese (I substituted a cheddar blend for cost and convenience)
(I also added an additional 1 cup of the same cheddar blend, because there's no such thing as too much cheese)
Salt and pepper to taste

Preheat the oven to 425 degrees F.

Melt the butter in a heavy-bottomed saute pan over medium heat. Add the onions and saute until translucent, about 15 minutes. Add the flour and nutmeg and cook, stirring, for 2 more minutes. Add the cream and milk and cook until thickened. Squeeze as much liquid as possible from the spinach and add the spinach to the sauce. Add 1/2 cup of the Parmesan cheese (and 1 cup Cheddar blend) and mix well. Season to taste with salt and pepper.

Transfer the spinach to a baking dish and sprinkle the remaining 1/2 cup Parmesan and the Gruyere (Cheddar) on top. Bake for 20 minutes until hot and bubbly. Serve hot.