1. For the topping: Process the bread and butter in a food processor until coarsely ground, about six 1-second pulses; set aside.

2. For the filling: Adjust an oven rack to the middle position and heat the oven to 400F/200C.  Bring 2 quarts/liters of water to a boil in a large pot.  Add 1/2 T salt and the broccoli to the boiling water; cover and cook until bright green and crisp-tender, about 3 minutes.  Drain the broccoli and leave it in the colander; set aside.

3. Meanwhile, melt the butter in a medium saucepan over medium heat.  Stir in the garlic, mustard, and cayenne; cook until fragrant, about 30 seconds.  Add the flour and cook, stirring constantly, until the flour turns golden, about 1 minute.  Slowly whisk in the milk and broth; bring to a simmer and cook, whisking often, until large bubbles erupt at the surface and the mixture is slightly thickened, about 5 minutes.  Off the heat, whisk in the cheeses.  Season to taste with salt and pepper.

4. To assemble and bake: Spread the broccoli in an 8" square baking dish (or shallow casserole dish of similar size).  Whisk the cheese sauce again briefly and pour over the broccoli.  Sprinkle with the bread-crumb topping.  Bake until golden brown and bubbling around the edges, about 15 minutes.  Cool for 5 minutes before serving.