Cover the potatoes with 2 inches water in a small saucepan; bring to a boil. Simmer 15 minutes or until fork-tender. Drain, halve, and transfer to a bowl. Immediately toss potatoes with mustard, 1 tablespoon lemon juice, 1 teaspoon olive oil, and salt and pepper, to taste.
Preheat oven to 400°.
Toss fennel in a bowl with remaining 2 tablespoons juice, remaining 1 teaspoon oil, onion, and 2 tablespoons fennel fronds.
Rinse fish, and pat dry. Arrange in a single layer, skin side down, on a small shallow baking pan. Combine sour cream, horseradish, and garlic in a bowl. Spread evenly on top of fish, and top with panko. Roast fish 7 minutes or until tops are golden brown and fish is just cooked through.
Divide potatoes among 4 serving plates with fennel salad, fish, and lemon wedges. Garnish with fennel fronds; serve.
