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 over medium-high heat, stirring, until thick. Stir in the Gruyere, season with salt and pepper and reserve.
Heat the oven to 425 degrees F/210 degrees C.
Make the gratin: Bring a large pot of water to the boil. Salt it, as for pasta, and blanch the cauliflower until just shy of being tender, about 5 minutes. Drain well. Pat dry and 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 gratineed on top, 5 to 10 minutes.