Broccoli and Gruyere Gratin

I made this dish to take to a big Thanksgiving feast this year and people raved about it. It's rich and thick and creamy and hearty and very flavorful.

Though the recipes calls for broccoli, I used broccolini and I think it added something special to the dish.
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Broccoli(ni) and Gruyere Gratin
*courtesy of Real Simple*

Serves 8

Ingredients:
2 large bunches broccoli, roughly chopped (or about 12 cups of broccolini)
4 tablespoons butter
1/4 cup all-purpose flour
2 cups whole milk
2 cups grated Gruyere (8 ounces)
kosher salt and black pepper

Directions:
1. Heat oven to 375 degrees. Fill a large saucepan with 1 inch of water and fit with a steamer basket (or fill a large pot with 1/2 inch of water). Bring to a simmer. Place the broccoli in the steamer basket, cover, and steam until just tender, 3 to 4 minutes; transfer to a large bowl.

2. Melt the butter in a medium saucepan over medium heat. Add the flour and cook, stirring, for 2 minutes (do NOT let it darken). Whisk in the milk and simmer until slightly thickened, 3 to 4 minutes. Remove from heat and stir in 1 cup of the cheese, 1/2 teaspoon salt, and 1/4 teaspoon pepper. Toss with the broccoli.

3. Transfer the broccoli mixture to a shallow 3-quart baking dish and sprinkle with the remaining cup of cheese. Bake until bubbling and golden brown, 35 to 40 minutes. Let sit 10 minutes before serving.

TIP for make-ahead: Prepare gratin, but do not bake. Refrigerate in the baking dish for up to 1 day in advance (alternatively, keep the gratin at room temperature for up to 2 hours). To serve, bring to room temperature and bake according to recipe.