Pour all the sauce over the penne in the pot and stir over medium low heat until heated through for serving. Add more pasta water at this stage if the sauce starts to get too thick or chalky. Stir in some of the cauliflower florets and coat in sauce and leave some remaining florets to garnish the final dish. Serve with more roasted cauliflower on top, parsley, and more ground black pepper to taste.
If you don't have mellow white miso and something like red or another version that is darker you can still use it. Those variations might have a stronger flavor so start with half the amount and taste it first. If you don't have miso at all you can substitute gluten-free tamari or low-sodium soy sauce!