Preheat the oven to 350°. Bring 2 large saucepans of salted water to a boil. Cut the cauliflower into 2 1/2-inch florets. Add the florets to the saucepans, cover and cook over moderate heat until just tender when pierced, about 6 minutes. Drain well. Arrange the cauliflower in a 9-by-13-inch (3 quart) baking dish.
Meanwhile, in a medium saucepan, combine the milk and bay leaf and bring just to a simmer over moderately low heat.
In another medium saucepan, melt the butter over moderate heat. Add the flour and cook, stirring, for 2 minutes. Whisk in one-third of the hot milk until smooth. Gradually whisk in the remaining milk and boil for 2 minutes, whisking frequently. Remove the bay leaf.
Whisk the mustard into the sauce. Season with the white pepper and a few gratings of nutmeg. Set aside 1 1/2 cups of the grated Gruyère and add the remainder to the sauce. Remove from the heat and stir to melt the cheese. Season with salt.
Pour the cheese sauce over the cauliflower. Toss the reserved cheese with the bread crumbs and sprinkle the mixture evenly over the top. Bake for about 45 minutes, until bubbling and golden.