To make Bechamel: In a heavy-bottomed 2-quart saucepan, melt butter over medium-low heat. When butter starts to foam, add flour and mix using a wooden spoon to create a smooth, loose paste. Cook, stirring constantly, allowing butter and flour to froth together until the mixture turns a light, golden sandy color, about 4 to 5 minutes. Remove pan from heat and let stand.

Meanwhile, combine milk and 1/4 teaspoon salt in a medium saucepan, scald milk (heating it until just below boiling point). Return saucepan with roux to medium-low heat. Add half of the hot milk to the roux, whisking vigorously to thoroughly blend and prevent lumps, and bring to a boil over medium-high heat. When a boil is reached and sauce is smooth, add the remaining milk and bay leaf, reduce heat to low, and simmer, whisking occasionally, until the sauce is smooth and thickened, about 10 to 12 minutes. Strain sauce through fine-mesh strainer. Season with salt, white pepper and nutmeg to taste, and set aside until ready to use. (Makes about 2 cups.)

To make Gratin: Preheat oven to 375 degrees F. Bring 3 quarts of water to a rapid boil with 2 tablespoons of salt and vinegar. (The acid in vinegar helps keep cauliflower white.) Cut cauliflower into 1 1/2-inch wide florets. Boil florets until just tender, about 4 to 5 minutes, drain well without rinsing, pat dry, and season with salt to taste.

Arrange the cauliflower, florets-side up, in a buttered, medium (about 8-cup) oval gratin dish or baking dish just large enough to hold cauliflour in a single layer. If the bechamel sauce is cold, bring it to a simmer on the stove while whisking. Gradually stir Gruyere cheese into the hot bechamel and adjust seasoning with salt and pepper, to taste. In a small bowl, combine the parmesan, bread crumbs and basil, and mix with olive oil, to coat evenly. Spoon bechamel over the cauliflower and sprinkle everything, evenly, with crumb topping.

Bake in center of oven until gratin is bubbling and lightly browned on top, about 30 to 35 minutes. Let rest for 5 to 10 minutes before serving. Serve hot, or at room temperature.