Combine potatoes, diced tomatoes, onion, butter, garlic, and salt in a stockpot; bring to a boil. Reduce heat, cover, and simmer, stirring every 10 minutes, for 25 minutes. Remove the lid and continue cooking for 10 minutes more.
Stir cashews and spinach into potato mixture; cook until spinach is lightly wilted, about 3 minutes.
Transfer potato mixture to a serving dish; top with Cheddar cheese, ranch dressing, and hot sauce.