1. Preheat the oven to 450°. On a work surface, combine the parsley, spearmint, lemon zest and garlic and chop together until blended. Transfer the gremolata mixture to a bowl and stir in the canola oil.
2. In a large bowl, toss the sweet potatoes with the olive oil, cayenne and salt. Spread the potatoes in a single layer on 2 large rimmed nonstick baking sheets. Roast on the upper and middle racks of the oven for 45 minutes, or until tender and browned in spots; shift the pans halfway through to ensure even cooking. Mound the fries on a platter and pass the gremolata separately.
Make Ahead: The oven fries and gremolata can be refrigerated overnight. Bring the gremolata to room temperature. Spread the fries on the baking sheets in a single layer and reheat in a 450° oven before serving.