Heat the oven to 425 degrees. On a large sheet pan, spread out the French fries in a single layer and scootch to the edges of the pan. Bake for 15 to 20 minutes, until slightly crispy at the edges.
Meanwhile, in a bowl, stir to combine the mayonnaise,capers, vinegar, 2 tablespoons dill, 1 teaspoon salt and 1/2 teaspoon black pepper. Transfer about half of the sauce to a small serving dish; set aside for later. Add the fish to the rest of the marinade and toss to coat. Cover and refrigerate until ready to cook
In a separate bowl, stir to combine the panko, olive oil,remaining 2 tablespoons dill, 1/4 teaspoon salt and 1/4 teaspoon black pepper.
When the fries are slightly crisp at the edges, add the marinated fish pieces to the sheet pan. Sprinkle the fish evenly with panko, then douse the fish and fries with vinegar to taste (1 to 2 tablespoons) and sprinkle with salt.
Bake the fish along side the fries until the fish is flaky and no longer opaque in the center, 12 to 15 minutes. Serve immediately with the reserved sauce.