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Preheat oven to 375 degrees F.
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Divide cauliflower into large florets and cook in boiling water until cauliflower is al dente (tender but firm). This will take 5 or 6 minutes. Remove from water and drain. Set aside.
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Melt butter in a medium saucepan and whisk in flour, stirring constantly for about two minutes. Pour hot milk into the butter and whisk until smooth and thick. This will happen quickly. Remove thickened sauce from heat and whisk in salt, black pepper, nutmeg, 1/2 C. Gruyere cheese, and all of Asiago cheese.
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Pour one third of sauce into a large 8 x 11 x 2 inch baking dish. Arrange drained cauliflower on top of sauce and cover with the remainder of the sauce.
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Combine bread crumbs with the remainder of the Gruyere cheese and sprinkle this mixture on top of the cauliflower. Melt remaining two tablespoons of butter and drizzle over the bread crumbs.
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Bake in a preheated 375 degree F. oven for 25-30 minutes or until the top of the gratin is browned.
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Serve cauliflower gratin hot or at room temperature.