Preheat your oven to 425 degrees. Place the cauliflower florets and the whole garlic cloves on a large rimmed baking sheet. Drizzle with 2 tablespoons of olive oil and sprinkle with 1/2 teaspoon of salt and 1/4 teaspoon of black pepper. Toss everything together so the florets are evenly coated, then spread them into a single layer with space between pieces. Crowding the pan causes steaming instead of roasting.

Roast at 425 degrees for 22 to 25 minutes, flipping the cauliflower once halfway through, until the florets are golden brown on the edges and the garlic cloves are soft and caramelized. The cauliflower should have some color and be just fork-tender, not mushy. Transfer everything to a lightly greased 2-quart baking dish and reduce the oven temperature to 375 degrees.

While the cauliflower roasts, make the cream sauce. Melt 2 tablespoons of butter in a medium saucepan over medium heat. Whisk in the flour and cook, stirring constantly, for 1 minute. Slowly pour in the milk and heavy cream while whisking continuously to prevent lumps. Cook over medium heat, whisking often, for 4 to 5 minutes until the sauce thickens enough to coat the back of a spoon.

Remove the saucepan from the heat. Add the shredded cheddar, dry mustard powder, the remaining 1/2 teaspoon of salt, and the remaining 1/4 teaspoon of black pepper. Stir until the cheese is fully melted and the sauce is smooth. Taste and adjust seasoning if needed. Pour the sauce evenly over the roasted cauliflower and garlic in the baking dish. The garlic cloves will be soft enough to press gently with the back of a spoon and they will melt into the sauce as it bakes.

In a small bowl, combine the breadcrumbs with 1 tablespoon of melted butter and the remaining 1 tablespoon of olive oil. Stir until all the crumbs are lightly coated. Scatter them evenly over the top of the gratin.

Bake at 375 degrees for 18 to 22 minutes, uncovered, until the sauce is bubbling around the edges and the breadcrumb topping is deep golden brown. If the top browns too quickly before the sauce bubbles, tent loosely with foil for the last 5 minutes. Let the gratin rest for 5 minutes before serving. Scatter parsley over the top if you like, and serve straight from the baking dish.