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