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1
Preheat the oven to 375 degrees F.
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2
Prepare a large bowl filled with ice water and ice.
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3
Cook the cauliflower florets in a large pot of boiling salted water for 5 to 6 minutes, until tender but still firm.
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4
Drain.
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5
Throw the cooked cauliflower into the bowl of ice water to stop them from cooking any further.
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6
Bring the pot back up to a boil and put in the quartered Brussels sprouts and cook for 3-4 minutes.
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7
While the sprouts are cooking, the cauliflower should be completely cooled.
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8
Drain them and set aside.
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9
After the Brussels sprouts are done, throw them in the bowl of ice water and ice.
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10
Meanwhile, melt 2 tablespoons of the butter in a medium saucepan over low heat.
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11
Add the flour, stirring constantly with a wooden spoon for 2 minutes.
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12
Pour the hot milk into the butter-flour mixture and stir until it comes to a boil.
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13
Boil, whisking constantly, for 1 minute, or until thickened.
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14
Off the heat, add 1 teaspoon of salt, pepper, nutmeg, 1/2 cup of the Gruyere, and the Parmesan.
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15
Pour 1/3 of the sauce on the bottom of an 8 x 11 x 2-inch baking dish.
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16
Place the drained cauliflower and brussels sprouts on top and then spread the rest of the sauce evenly on top.
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17
Combine the bread crumbs with the remaining 1/4 cup of Gruyere and sprinkle on top.
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18
Melt the remaining 2 tablespoons of butter and drizzle over the gratin.
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19
Sprinkle with salt and pepper.
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20
Bake for 25 to 30 minutes, until the top is browned.
