For the Bechamel
- 2 tablespoons unsalted butter
- 2 tablespoons all-purpose flour
- 2 cups whole milk
- Coarse salt and freshly ground pepper
- 2/3 cup grated smoked Gouda cheese (2 ounces)
For the Brussels Sprouts
- Coarse salt
- 1 1/2 pounds brussels sprouts
For Assembling the Gratin
- 2/3 cup finely grated aged Gouda (2 ounces)
- Smoked flaked sea salt, such as Maldon (optional) or regular sea salt
- Preheat oven to 375 degrees. Make the bechamel: Melt butter in a saucepan over medium heat. Add flour and whisk until mixture bubbles slightly but has not started to brown, about 2 minutes. Gradually whisk in milk. Raise heat to medium-high and bring to a boil, whisking often. Reduce heat to low and cook, stirring occasionally with a wooden spoon, until thickened, 12 to 15 minutes. Season with coarse salt and pepper. Remove from heat. Add smoked Gouda and stir until melted.
- Meanwhile, blanch the brussels sprouts: Bring a medium pot of salted water to a boil. Cook brussels sprouts until just tender when pierced with the tip of a paring knife, 3 to 4 minutes. Drain and transfer to an 8-cup (8-by-12-inch) baking dish.
- Assemble the gratin: Pour bechamel over brussels sprouts and sprinkle with aged Gouda and a pinch of smoked sea salt. Bake, uncovered, until bubbling and golden, about 25 minutes.
Bechamel can be refrigerated for up to 4 days. Cooled blanched brussels sprouts can be refrigerated for up to 1 day. Assembled gratin can be refrigerated overnight. Bring to room temperature before baking.