Heat oven to 400° F. Cook the pasta according to the package directions, adding the cauliflower during the last 3 minutes of cooking time; drain.
Meanwhile, pulse the bread in a food processor until coarse crumbs form. Add the parsley, 2 tablespoons of the oil, and ¼ teaspoon each salt and pepper; pulse to combine.
Return the pasta pot to medium heat and add the remaining tablespoon of oil. Add the onion, ¾ teaspoon salt, and ½ teaspoon pepper and cook, stirring occasionally, until soft, 3 to 5 minutes.
Mix in the pasta and cauliflower and the cheese, sour cream, milk, and mustard.
Transfer to a shallow 3-quart baking dish, sprinkle with the bread crumbs, and bake until golden brown, 12 to 15 minutes.
