Step 1

Preheat the oven to 425˚. Generously butter a 3-quart baking dish. Spread the cauliflower on a rimmed baking sheet in an even layer and roast until just starting to brown around the edges but still a bit firm, about 20 minutes. Remove from the oven and set aside until no longer steaming, about 5 minutes. Reduce the oven temperature to 375˚.

Step 2

Meanwhile, melt 3 tablespoons butter in a medium saucepan over medium-low heat. Add the flour and stir to combine. Let cook until bubbling, about 1 minute. Slowly whisk in the milk and heavy cream. Cook, stirring, until the mixture is thick enough to coat the back of a spoon and comes to a gentle simmer, 6 to 8 minutes. Reduce the heat to low; add the salt and three-quarters of the gruyère and stir until the cheese is melted and the sauce is smooth, about 1 minute.

Step 3

Melt the remaining 3 tablespoons butter in a small skillet over low heat. Add the rosemary and thyme and cook until bubbling, about 30 seconds. Add the panko and stir to coat completely in the butter. Remove from the heat and stir in the parmesan and remaining gruyère.

Step 4

Transfer the cauliflower to the baking dish. Pour the sauce over the cauliflower and sprinkle the panko mixture on top. Bake until the panko is golden brown and the cauliflower is tender, about 25 minutes. Let rest for 15 minutes before serving.