Make the sauce: Bring the milk to a simmer in a saucepan with the bay leaf and garlic. Cover, turn off the heat, and let infuse, about 15 minutes. Melt the butter in a saucepan. Whisk in the flour and cook for a minute to remove the raw flour taste. Gradually whisk in the milk. Cook, stirring, until thick. Stir in the cheese, season with salt and pepper, and reserve.
Heat the oven to 425°F/210°C. Bring a large pot of water to the boil. Salt it, as for pasta, and blanch cauliflower until just shy of being tender, about 5 minutes. Drain well. Pat dry with a tea-towel. Season with salt and pepper. Toss with the sauce and dump into a gratin dish. Scatter over the breadcrumbs and Parmesan and bake until bubbling and gratinéed on top.