2 pints (about 1 1/2 pounds) Brussels sprouts, trimmed
3 tablespoons unsalted butter
3 tablespoons all-purpose flour
2 cups milk, at room temperature
5 ounces (about 1 cup) Gruyère cheese, grated
Kosher salt and freshly ground black pepper, to taste
1. Preheat the oven to 400°F (204°C).
2. Fill a large bowl halfway with ice water. Steam or boil the Brussels sprouts until bright green and beginning to soften but not mushy, 3 to 5 minutes, depending on their size. Drop the sprouts into the ice water to stop the cooking, then drain and pat them dry. (You can set the sprouts aside for up to several hours before continuing with the recipe.)
3. Melt the butter and flour in a saucepan over medium heat. Cook, stirring, until smooth and bubbling, about 1 minute. Whisking constantly, slowly stir in the milk. Continue to cook, whisking frequently, until thick and creamy, 2 to 3 minutes. Add 3/4 cup of the cheese and stir until melted and smooth. Remove the pan from the heat and season the sauce well with salt and heavily with pepper.
4. Halve the sprouts through the core or, if they’re large, quarter them. Place the sprouts in an even layer in a 2-quart baking dish, pour the sauce evenly over the top, and sprinkle with the remaining cheese. Bake until golden and bubbling on top, 10 to 15 minutes. If the cheese doesn’t turn quite golden enough, run the gratin under the broiler for a few seconds. Serve hot, hot, hot.