For the sweet potato wedges:
1 1/2 pounds sweet potatoes, scrubbed
2 tablespoons vegetable or canola oil
3/4 teaspoon kosher salt
For the fish fingers:
Cooking spray or vegetable oil
1/2 cup mayonnaise
1 tablespoon Dijon mustard
1 tablespoon finely chopped fresh parsley
Freshly ground black pepper
1 1/4 cups panko breadcrumbs
1 pound fresh cod or tilapia fillets
Lemon wedges, for serving (optional)
For the sweet potato wedges: Arrange two racks to divide the oven into thirds and heat the oven to 400°F. Place a baking sheet on the bottom rack while the oven is heating.
Cut the sweet potatoes in half lengthwise. Cut each piece lengthwise into 1/2-inch-wide wedges. (If the wedges are really long, cut them in half.) Place in a large bowl. Drizzle with the oil, sprinkle with the salt, and toss to evenly coat.
Carefully remove the hot baking sheet out of the oven. Add the sweet potatoes and arrange into an even layer. Bake until golden-brown on the bottom, about 20 minutes. Meanwhile, prepare the fish.
For the fish fingers: Line a baking sheet with foil and spray the foil with cooking spray or grease with oil.
Mix the mayonnaise, mustard, and parsley together in a small bowl. Taste and season with salt and pepper as needed. Place the panko in a shallow bowl or pie dish. Cut the fish fillets lengthwise into 3/4-inch-wide "fingers" that are about 4 to 5 inches in length. Generously season all sides of the fish with salt and pepper.
Brush the mayonnaise mixture onto all sides of the fish. Dip the fish into the panko on all sides, pressing it in with your fingers and making sure it is completely coated. Place the breaded fish in a single layer on the baking sheet, giving them as much room as possible between pieces.
When the sweet potatoes are golden-brown on the bottom, remove the baking sheet from the oven and flip the wedges over with a flat spatula. Place the sheet back onto the bottom rack and place the baking sheet with the fish on the top rack.
Bake until the fish is no longer opaque and cooked through, and the sweet potato wedges are tender, 12 to 15 minutes more. Serve with lemon wedges, if using.