-
1
Heat the oven to 350 degrees F and arrange a rack in the middle.
-
2
Coat a 13-by-9-inch baking dish with butter; set aside.
-
3
Fill a large saucepan with generously salted water and bring to a boil over high heat.
-
4
Add the cauliflower and cook until just knife tender, about 5 minutes.
-
5
Drain in a colander; set aside.
-
6
Return the saucepan to medium heat and melt 4 tablespoons of the butter until foaming.
-
7
Add the shallots and cook, stirring occasionally, until softened, about 4 minutes.
-
8
Reduce the heat to low, add the flour, and whisk until smooth and the raw taste has cooked off, about 1 minute.
-
9
Gradually whisk in the milk.
-
10
Add the mustard, Worcestershire, bay leaf, cayenne, and measured salt and season generously with pepper.
-
11
Cook, stirring often, until the sauce has just thickened, about 10 to 12 minutes.
-
12
Discard the bay leaf.
-
13
Add the reserved cauliflower to the sauce and stir to coat.
-
14
Transfer to the prepared baking dish and arrange in an even layer.
-
15
Bake until bubbly and the edges start to brown, about 35 to 40 minutes.
-
16
Meanwhile, heat the remaining 3 tablespoons butter in a medium frying pan over medium heat.
-
17
When the foaming subsides, add the panko and season with salt and pepper.
-
18
Cook, stirring occasionally, until lightly browned and toasted, about 6 to 8 minutes.
-
19
Transfer to a medium bowl; set aside.
-
20
When the casserole is done, let it sit for 5 to 10 minutes.
-
21
Sprinkle the toasted panko evenly over top and serve.
