Crispy Fish with Smashed Peas and Oven Fries

Heat oven to 425 degrees F. On a large rimmed baking sheet, toss the potatoes with 2 tablespoons oil and 1/2 teaspoon each salt and pepper. Roast for 12 minutes. In a small bowl, combine the bread crumbs, parsley, and 1 tablespoon oil. Season the fish with 1/4 teaspoon each salt and pepper. Push the potatoes to the edges of the pan and place the fish in the center. Spread the mustard over the fish and top with the bread crumb mixture. Roast until the potatoes are tender and the fish is opaque throughout, 10 to 12 minutes more. While the fish is cooking, bring a small saucepan of water to a boil. Add the peas and boil until heated through, about 2 minutes. Drain and return them to the pan. Add the horseradish, remaining 2 tablespoons oil, and 1/4 teaspoon each salt and pepper, and gently smash. Serve with the fish and fries.