The best thing about perfectly roasted potatoes is that crisp, crunchy exterior and fluffy, flavorful insides. If you're looking for a method that delivers every time, a quick parboil before you roast the potatoes in the oven is the easiest way to get it.
Image from ThinkYum.
Parboiling your potato slices before roasting helps cook them evenly, and helps them crisp up quickly. Kenji Lopez-Alt, writing at Cooking Light, explains how this method works:
Simmering starchy baking potatoes in water (seasoned with salt) gelatinizes a layer of starch on their exterior surfaces. This layer will then dehydrate and brown as the potato roasts, creating a thick, crisp shell.
Once the potatoes are tender, remove them from the water. Then dress as you normally would with oil and spices and pop them in the oven. You don't want to overboil the potatoes though, so he suggests you add a tablespoon of white vinegar to the water to stop the potatoes from falling apart.
This tip isn't just useful for roasted potato slices either: You can apply this method to home fries, smashed potatoes, steak fries, or just about any other baked potato dish.
Help Me, Kenji — How to get roasted potatoes extra crispy [Cooking Light]