Sous Vide Cauliflower Gratin

Preheat the water bath to 85°C (185°F).

Prepare the cauliflower by trimming away any outer leaves or discolored stem but leaving the core intact. Place the cauliflower on a cutting board and slice into 1-to 11⁄2-inch-thick slices. Season the cauliflower slices with salt.

Gently transfer the cauliflower into a 1-gallon freezer-safe ziplock bag or a vacuum seal bag, keeping the slices as whole as possible (if they fall apart somewhat, it’s okay). Arrange them in a single layer with as little overlap as possible to ensure even cooking.

Seal the bag using the water displacement method (page 12) or a vacuum sealer, adding weights to the bag as necessary (page 130) to ensure that it sinks.

When the water reaches the target temperature, lower the bagged cauliflower in the water bath (making sure the bag is fully submergeand cook for 25 minutes (or up to 1 hour).

Remove the bag from the water bath, let it rest in the bag for at least 10 minutes or up to 1 hour before proceeding.

If making ahead, transfer it to an ice water bath (page 18), and chill until completely cold, about 30 minutes. Once cooked and chilled, the cauliflower can be refrigerated in the bag for up to 2 weeks.

Preheat the oven to 450°F.

Remove the cooked cauliflower from the bag, discarding any liquid in the bag, and thoroughly pat dry with paper towels. Use a paring knife or just your hands, cut or break the cauliflower into bite-size pieces. Set aside.

Heat 2 tablespoons of the butter in a medium pot over medium-low heat until bubbling. Add the garlic and cook, stirring constantly with a whisk, until fragrant but not browned, about 30 seconds. Add the flour and continue to stir, cooking for 1 minute. Add the wine and then add the milk in a steady stream, whisking constantly to avoid lumps. Increase the heat to medium and bring the sauce to a boil, stirring frequently. Reduce heat to a simmer and cook for 5 minutes, until the sauce is thick enough to coat the back of a spoon.

Remove from the heat and stir in the nutmeg, thyme, and 1 cup of the Gruyère, whisking until smooth. Season to taste with salt and pepper, then stir in the cauliflower to coat.

Lightly grease an 8 by 8-inch baking dish or casserole with butter. Add the cauliflower and sauce mixture and spread it into an even layer with the back of a spoon or spatula. Sprinkle on the remaining 1⁄2 cup of Gruyère, followed by the bread crumbs. Dot with the remaining 1 tablespoon of butter, then transfer the dish to the oven. Bake for 20 to 25 minutes, until golden brown and bubbly. Sprinkle with the flaky sea salt and parsley. Let rest for 5 to 10 minutes before serving.