Purple potatoes cook much like regular potatoes, since you can boil, mash, bake, or roast them. 5-6 medium-sized purple or other potatoes 3 tablespoons extra virgin olive oil 1 clove garlic, crushed 1 1/2 teaspoons dried rosemary, crushed Kosher salt and freshly ground pepper to taste. P.S. These 7 easy purple potato recipes will make you a fan of this superior spud. Rosemary Roasted Purple Potatoes. Preheat oven to 375 degrees. It goes without saying you may make this recipe with any kind of potatoes.