Adapted from Ina Garten from Barefoot In Paris
When I first made this dish the roux was a bit tough for me to figure out. I didn’t quite understand why I was mixing melted flour with butter but it works as an amazing thickening agent for the milk and cheese sauce. I don’t use nearly as much nutmeg as the recipe initially called for because I’m not a huge fan of the intense flavor.
3/4 head of cauliflower, cut into large florets
Preheat the oven to 375 degrees F.
Prepare a large bowl filled with ice water and ice. Cook the cauliflower florets in a large pot of boiling salted water for 5 to 6 minutes, until tender but still firm. Drain. Throw the cooked cauliflower into the bowl of ice water to stop them from cooking any further. Bring the pot back up to a boil and put in the quartered brussels sprouts and cook for 3-4 minutes. The cauliflower should be completely cooled. Drain them and set aside. After the brussels sprouts are done, throw them in the bowl of ice water and ice.
Meanwhile, melt 2 tablespoons of the butter in a medium saucepan over low heat. Add the flour, stirring constantly with a wooden spoon for 2 minutes. Pour the hot milk into the butter-flour mixture and stir until it comes to a boil. Boil, whisking constantly, for 1 minute, or until thickened. Off the heat, add 1 teaspoon of salt, the pepper, nutmeg, 1/2 cup of the Gruyere, and the Parmesan.
Pour 1/3 of the sauce on the bottom of an 8 by 11 by 2-inch baking dish. Place the drained cauliflower and brussels sprouts on top and then spread the rest of the sauce evenly on top. Combine the bread crumbs with the remaining 1/4 cup of Gruyere and sprinkle on top. Melt the remaining 2 tablespoons of butter and drizzle over the gratin. Sprinkle with salt and pepper. Bake for 25 to 30 minutes, until the top is browned.