Preheat the oven to 425°F and coat a large, shallow baking pan with nonstick spray.

Place the cauliflower florets in a large bowl, drizzle with the oil and season with salt and pepper. Arrange in a single layer on the prepared baking pan and roast until tender and just beginning to turn golden, 18 to 20 minutes.

Remove the cauliflower from the oven, return to the bowl and set aside. Reduce the oven temperature to 350°F. Coat a 2-quart casserole with nonstick spray and set aside.

While the cauliflower is roasting, heat the milk in the microwave or a separate saucepan until very warm, but not scalded.

Melt 1 tablespoon of the butter in a saucepan over medium heat. Add the panko crumbs, stir to combine and transfer to a small bowl. Add 1/4 cup of the cheese and set aside.

Add the remaining 3 tablespoons of butter to the saucepan and melt over medium heat. Sprinkle with the flour and whisk until blended. Continue cooking, whisking continually, until the mixture is golden and smooth, 2 to 3 minutes.

Gradually add the warmed milk, whisking continually until well blended. Continue cooking until the sauce is smooth and thick enough to coat the back of a spoon, 5 to 6 minutes.

Reduce the heat to low, add the remaining cup of cheese and stir until completely melted. Pour the sauce over the cauliflower, combine well and transfer the mixture to the prepared casserole.

Top evenly with the panko-cheese mixture and bake uncovered until lightly browned and bubbly, 20 to 30 minutes.