I use a stainless steel mandolin to cut potatoes, yams and onions in thin slices for gratins. Using the guard is imperative, as mandolin blades are really sharp and an accident can happen very quickly. For fries, a good knife does the trick. Since I cut down on grains and other carbohydrates, I eat a lot of tubers like potatoes, yams, sweet potatoes, jicama, taro, cassava, jerusalem artichoke and the likes at practically every meal. The easiest way is to cube them, coat in olive oil, add whatever seasoning you want, toss on a baking sheet and roast in the oven. You can add chopped daikon, beet, onion and garlic, turnip and parsnip, etc.