Method:
Preheat oven to 350°F. Toss garlic cloves in olive oil and transfer to a glass baking dish. Roast, shaking the pan several times while cooking, until garlic is golden and soft, about 30 minutes.
Meanwhile, steam potatoes until soft, then transfer to a large bowl and mash. Put potatoes, roasted garlic, and milk into a food processor and puree. Stir in tarragon, lemon zest, 2 teaspoons of the lemon juice, and salt and pepper.
Preheat broiler. Arrange fillets in a single layer in a baking dish and cover each evenly with potato mixture. Squeeze remaining lemon juice liberally over each fillet. Broil fish for 12 to 15 minutes, until golden brown and opaque throughout. Serve hot, garnished with lemon wedges.