Prep the sweet potato - Peel the sweet potato and cut it into thin, round slices using a sharp knife or mandoline slicer.
Mix the batter - In a mixing bowl, combine the rice flour, cornstarch, baking powder, salt, and pepper. In a separate bowl, whisk together the egg and cold water, then slowly pour it into the dry ingredients, whisking constantly until the batter is smooth and free of lumps.
Heat the oil - Heat the oil in a deep pan or wok over medium-high heat until it reaches 350°F - 375°F.
Dredge the fish and sweet potato - Dredge the white fish and sweet potato slices in the batter, making sure they are evenly coated on both sides.
Fry the tempura - Working in batches, carefully drop the battered fish and sweet potato slices into the hot oil and fry until golden brown and crispy, about 2-3 minutes. Use a slotted spoon or metal tongs to transfer the tempura to a paper towel-lined plate.
Serve and enjoy! - Serve the hot tempura immediately as an appetizer, side dish, or main course with your favorite dipping sauce. Enjoy!