Preheat oven to 400 F. Butter a 1 1/2 quart gratin dish and set it aside.
Make the sauce. In medium sauce pan, heat the milk with the onion, bay leaf, and thyme to just below the boiling point. Set aside. In another medium pan, melt 2 tablespoons of the butter over medium heat, and then sift in the flour. Cook the mixture, stirring constantly with a wooden spoon in a figure-8 motion, until it lightens in color, about 2 minutes. Remove from the heat and cool slightly.
Gradually whisk the milk mixture into the flour mixture. Bring to a boil over medium high heat, whisking to prevent lumps. Reduce heat to low, and simmer, whisking occasionally, until the sauce is thickened, about 10 minutes.
While the sauce simmers, bring a medium pot of water to a boil, season with salt, and add the cauliflower. Cook until tender, about 4 to 5 minutes. Drain and pat dry. Arrange the cauliflower, florets-side up, in the buttered gratin dish.
Strain the sauce and season it with a teaspoon salt, pepper, nutmeg, and cayenne to taste. Whisk in the cheese, 1 tablespoon of the butter, and the Cognac until smooth, taking care not to over mix. Pour the cheese sauce over the cauliflower.
Melt the remaining 2 tablespoons butter in a small saucepan. In a medium bowl, mix the bread crumbs with the butter to coat evenly. Sprinkle the top of the cauliflower with the buttered breadcrumbs. Bake until the top of the cauliflower gratin is golden brown and bubbly, about 20 to 25 minutes. Let rest for 5 minutes before serving. Serve hot.
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