Preheat oven to 425º F. Toss cauliflower with olive oil, salt, and pepper.
Place cauliflower on a sheet tray and roast for 7-9 min. or until done, set aside.
Cook pasta according to package directions.
Meanwhile, in a skillet over medium heat add butter and onion. Cook for 1-2 min. Whisk in cream and mustard, bring to a simmer, whisk in cheddar and ½ of the Parmigiano, and cauliflower.
Place mixture in a blender and blend until smooth, then mix with pasta. Top with breadcrumbs, remaining Parmigiano cheese, and parsley.
