Place the coated potatoes in the hot oven to bake for 20 minutes. Then remove from the oven and flip them so the other flat side of the potato is now touching the pan. Roast for another 15 minutes.
Remove the roasted sweet potatoes from the oven. Add the fresh rosemary and smashed garlic. Then using a spoon, scoop the butter and pour it on the tops of the sweet potatoes.