Oven & Pan Prep: Position oven racks in the upper and lower thirds of the oven and preheat to 375°F (190°C). Line the two baking sheets with parchment paper to prevent sticking.

Vegetable Preparation: Set your mandoline to a very thin setting, approximately 1/16-inch (1.5 mm). Carefully slice the sweet potato, russet potato, and parsnips into uniform rounds. Uniformity is key for the crisps to bake evenly.

Starch Release Soak: Place the sliced potatoes and parsnips into a large bowl and cover with cold water. Let them soak for 20 minutes. This step is crucial for removing excess surface starch, which helps achieve a truly crispy texture.

Towel-Dry Thoroughly: Drain the soaked vegetable slices completely. Spread them out on a clean, dry kitchen towel or several layers of paper towels. Pat them down until they are as dry as possible; any residual moisture will steam the vegetables instead of crisping them.

Seasoning the Crisps: In a small bowl, whisk together the smoked paprika, garlic powder, onion powder, and optional cayenne. Transfer the dry vegetable slices to a large, dry mixing bowl and sprinkle the seasoning blend over them, tossing gently with your hands to coat each slice lightly.

First Bake: Arrange the seasoned vegetable slices in a single, even layer across the prepared baking sheets, ensuring they do not overlap. Place one sheet on each oven rack and bake for 10-12 minutes. The edges of the crisps should just begin to curl and their surfaces will appear dry to the touch.

Flip & Final Crisp: Remove the baking sheets from the oven. Working quickly, use tongs to flip each crisp over. Rotate the sheets, moving the top one to the bottom rack and the bottom to the top. Return them to the oven and bake for another 8-12 minutes, or until the crisps are lightly golden and snap easily once slightly cooled. Remove from the oven and let them cool completely on the baking sheets before serving.