No white potatoes here! Just sweet potatoes and cauliflower mashed until silky and amped up with milk, butter, and a touch of nutmeg and cayenne.
Kosher salt and freshly ground pepper
2 1/2 pounds sweet potatoes (4 medium), peeled and cut into 2-inch pieces
1 head cauliflower (2 pounds), cored and cut into large florets
2/3 cup whole milk
1 stick unsalted butter
Pinch of freshly grated nutmeg
Pinch of cayenne pepper
Bring a large pot of salted water to a boil. Add sweet potatoes and return to a boil, then reduce heat and simmer 8 minutes. Add cauliflower; return to a boil, then reduce heat and simmer until potatoes and cauliflower are very tender, 10 to 12 minutes. Drain and return vegetables to pot. Place over medium heat and cook, stirring, until a film forms on bottom of pot, 1 to 2 minutes.
Combine milk, butter, nutmeg, and cayenne in a small pot. Bring just to a simmer. Add to cauliflower-potato mixture and mash. Season with salt and pepper. Serve immediately, or keep warm in a heatproof bowl set over a pot of simmering water up to 1 hour. Mash can be rewarmed by adding about 1/4 cup milk and stirring over low heat.