Place potatoes in a single layer on a large baking sheet coated with cooking spray. Brush evenly with oil. Sprinkle rosemary and next 3 ingredients over potatoes. Bake at 425° for 30 to 35 minutes or until potatoes are lightly browned, turning once.
Combine mayonnaise and next 3 ingredients. Brush both sides of fish fillets with mixture. Combine breadcrumbs, 1 teaspoon pepper, and garlic powder; dredge fish in breadcrumb mixture. Place on a baking sheet coated with cooking spray. Bake at 425° for 25 minutes. Serve fish and potato with Radish Tartar Sauce.