Remove the tips from the potatoes. Slice them using a very thin setting on a mandolin slicer. I use the 0.5mm setting on my mandolin. Place the slices into a bowl of cold water as you go. Remove the tips from the beets and discard the greens. Slice them and place them in a separate bowl of cold water.
Heat the oil in a large cast iron Dutch oven over high heat. A Dutch oven is perfect for frying because it holds a lot of heat which minimizes the cooling of the oil with each batch of chips. Heat the oil to 350°F. Use a candy/oil thermometer clipped to the side of the pot to monitor the temperature.
Fry the sweet potatoes separately from the beets in small batches so that you don’t overload the oil. Transfer a small batch of slices from the cold water to the hot oil, being careful to drip as little water as possible into the oil. Fry them until the bubbling stops or significantly slows, then remove immediately with a wire basket spoon. Each batch should require less than a minute. Spread the cooked chips onto a wire rack topped with paper towels. While the next batch is cooking, season the cooling batch with sea salt.
Once cool, keep the chips in an airtight container until serving time.
