Preheat the oven to 375 degrees F and butter a 9x13 inch (3 quart) casserole dish.
Fill a large pasta pot with water and bring to a boil.
Heat the milk either in the microwave or on the stove.
Melt the butter in a large saucepan, then add the flour and stir for about 1 minute.
Slowly pour in the hot milk while vigorously whisking. Continue whisking until the mixture bubbles and thickens, about 2 minutes.
Remove the pan from the heat, then stir in the salt, nutmeg, black pepper, cayenne pepper, and all but 2 cups of cheese (a mixture of all 3).
When the water is boiling, add the pasta and cook for exactly 6 minutes (set a timer).
Add the cauliflower to the boiling water with the pasta, stir, and cook for another 3 minutes.
Drain the pasta and cauliflower well, then combine with the cheese sauce and pour it into the prepared casserole dish.
Sprinkle the remaining 2 cups of cheese, then scatter the breadcrumbs on top.
Bake for about 30 minutes, until golden brown and bubbling on top. Allow to rest for 10 minutes before serving.