Cook: Roughly chop cauliflower into marble-sized pieces. Bring a large pot of water to a boil, then add cauliflower and salt. Cover and let simmer for about 5 minutes, or until cauliflower is fork-tender.
Stir: Remove from heat and drain cauliflower very well, then add back into the pot. Add cheddar cheese, cream cheese, and mustard, stirring until cheese has fully melted. Taste and add salt and pepper as needed.
For a melty browned top, transfer cooked cauliflower macaroni and cheese to a baking dish. Sprinkle with a bit more cheddar and broil in the oven for just a few minutes, until cheese has melted and is golden brown.