Heat your oven to 425 degrees F. Spread the cauliflower florets on a large rimmed baking sheet, drizzle with 2 tablespoons of the olive oil, sprinkle with 1/2 teaspoon salt and 1/4 teaspoon pepper, and toss everything to coat. Spread the florets in a single layer with some space between them. Roast for 22 to 25 minutes, flipping once halfway through, until the edges are golden brown and the florets feel tender when poked with a fork. Pull them out and drop the oven temperature to 375 degrees F.

While the cauliflower roasts, make the cheese sauce. Melt the butter in a medium saucepan over medium heat. Add the minced garlic and cook for about 60 seconds, stirring constantly, until fragrant but not browned. Whisk in the flour and keep whisking for about 90 seconds so the raw flour taste cooks out. The mixture will look like a pale paste.

Slowly pour in the warm milk while whisking steadily, then add the heavy cream. Keep whisking and cook over medium heat for 4 to 5 minutes until the sauce thickens enough to coat the back of a spoon. If you drag your finger across the spoon, the line should hold. Remove the pan from the heat.

Stir in the grated white cheddar a small handful at a time, letting each addition melt before adding more. Stir in the Dijon mustard, nutmeg, the remaining 1/2 teaspoon salt, and the remaining 1/4 teaspoon pepper. Taste the sauce and adjust seasoning if needed. It should taste boldly cheesy and slightly sharp.

Transfer the roasted cauliflower to a 2 quart baking dish and spread it into an even layer. Pour the cheese sauce over the top and use a spoon to nudge it down into the gaps between florets.

Mix the panko breadcrumbs with the remaining 1 tablespoon olive oil and the thyme leaves in a small bowl until the crumbs are lightly coated. Scatter the breadcrumb mixture evenly over the surface of the gratin.

Bake at 375 degrees F for 20 to 25 minutes until the sauce is bubbling around the edges and the panko topping is deep golden brown. Let the gratin rest for 5 minutes before serving so the sauce settles slightly. Serve straight from the baking dish.