Bring a large pot of salted water to a boil. Add the pasta and cook according to package directions until al dente. Drain and set aside.

Meanwhile, place the cauliflower, milk, bay leaf and garlic in a medium saucepan. Simmer until cauliflower is tender, 10-12 minutes, stirring often to prevent the milk from catching bottom. Remove the bay leaf and discard.

Scoop the cauliflower and garlic out of the saucepan with a slotted spoon and transfer them to a blender. Add 1½ cups of the warm milk (reserve the rest of the milk). Carefully blend the mixture until smooth. Pour the mixture back into the saucepan and stir in the cheeses, mustard powder, salt and pepper over medium heat until the cheese is melted. Add the reserved milk if needed to thin out the sauce. Taste and adjust seasoning as desired. Add the pasta to the sauce and stir until it is all coated with the sauce.

If desired, you can serve the mac and cheese at this point. If you are making the topping, mix the breadcrumbs, parmesan, parsley and oil together in a small bowl. Transfer the mac and cheese to an 8-inch square baking dish sprayed with nonstick cooking spray. Sprinkle the breadcrumb mixture evenly over the top. Bake in the oven for 15-20 minutes until it is heated through and the topping is nicely browned. Serve hot.