Bring a large pot of water to a boil over medium heat. Salt the water and add the broccoli and cauliflower florets. Boil the vegetables for 5 minutes, then remove them with a spider or a strainer and drain. Add the pasta to the water and undercook by about 2 minutes, then drain.
Meanwhile, combine the sour cream in a large bowl along with the mustard, chives, garlic, hot sauce and salt and pepper, to taste. Add the pasta and cauliflower and two-thirds of the cheese. Stir to combine, then transfer to a casserole or baking dish and cover with the remaining cheese. Cool and chill for a make-ahead meal.
To heat and eat, place the casserole on baking sheet and bake it in the middle of a pre-heated 375°F oven until deeply golden and bubbly, about 40-45 minutes.