Prep and roast the potatoes and onions: Preheat oven to 375˚ F. Place the potato and onion wedges on a baking sheet with sides. Drizzle over olive oil then rub all over each piece with your hands. Season with a sprinkle of salt and a few grinds of black pepper. Place in the oven until the potatoes and onions are tender and golden, about 30 to 40 minutes.
Blend to serve: Remove the baking sheet from the oven and pour all the contents including the juices, into a food processor. Add the butter and blend until smooth. Taste and season with more salt, if needed.