Position racks in the upper and lower thirds of the oven and preheat to 450 degrees . In a small bowl, stir together the sour cream, mayonnaise, lemon juice, shallot, 1/2 teaspoon salt and 1/4 teaspoon pepper. Cover and refrigerate until ready to serve.
On a foil-lined baking sheet, toss the sweet potatoes with the olive oil and 1 tablespoon parsley; season with salt and pepper. Spread in a single layer and bake on the lower rack until tender and crisp around the edges, about 20 minutes.
Meanwhile, coat another foil-lined baking sheet with cooking spray. In a medium bowl, whisk together the cornmeal, breadcrumbs, the remaining 2 tablespoons parsley, 3/4 teaspoon salt and 1/4 teaspoon pepper. In another bowl, combine the buttermilk and fish, turning to coat. Working with 1 piece at a time, coat the fish with the cornmeal mixture; transfer to the prepared baking sheet. Spray the fish with cooking spray; bake on the upper rack until golden-brown, 12 to 14 minutes. Serve with the fries, dipping sauce and lemon wedges.