How to Make It
Put oven rack in top third of oven and preheat to 450°. Put cauliflower florets, cut no larger than 1 in., on a rimmed baking sheet. Toss with oil and 1/2 tsp. each salt and pepper until evenly coated.
Spread in a single layer and roast until cauliflower is very tender and brown on the edges, turning occasionally, about 25 minutes. Set aside about 1/3 cup of the best-looking florets, then coarsely mash remaining florets on baking sheet with a potato masher.
Put potatoes in a large pot and add enough cold water to barely cover. Stir in 1 tsp. salt. Bring to a boil over high heat, covered. Reduce heat to medium and boil gently until potatoes are tender when pierced, about 15 minutes. Drain; return to pot, off the heat.
Mash hot potatoes with a potato masher until all large chunks are broken and potatoes are fluffy. Add milk, 2 tbsp. butter, 1 tsp. salt, and pepper; mash to blend. Stir in mashed cauliflower and salt and pepper to taste. Transfer to a serving bowl.
Top hot potatoes with remaining 1 tbsp. butter and reserved roasted cauliflower florets.
Make ahead: Up to 1 day, chilled. Reheat in a microwave until hot (if they've stiffened up, add enough milk to make them creamy).