Preheat the oven to 425°F (220°C). Line a large baking sheet with parchment paper for easy cleanup and to help prevent sticking.
Wash and peel the sweet potatoes if desired. Cut them into evenly sized wedges or 1/2-inch cubes so they cook at the same rate.
Place the cut sweet potatoes in a large bowl and sprinkle the cornstarch over them. Toss gently to coat evenly — this step is the secret to extra crispiness.
Add the olive oil, kosher salt, black pepper, smoked paprika, and garlic powder to the bowl. Toss until every piece is lightly and evenly coated with oil and seasonings.
Spread the potatoes out in a single layer on the prepared baking sheet, leaving space between pieces. Crowding the pan steams the potatoes and reduces crisping.
Roast in the preheated oven for 20 minutes, then use a spatula to flip the pieces. Return to the oven and roast another 10–15 minutes, or until the edges are browned and crisp and the centers are tender.
Remove the baking sheet from the oven. Drizzle the lemon juice over the hot potatoes and toss gently. Sprinkle chopped parsley on top and taste, adjusting salt if needed.
Serve immediately for the best texture. These are delicious on their own, as a side to roasted chicken or fish, or tossed into grain bowls and salads.