1. Adjust oven rack to middle position and heat oven to 325°F. Combine parsley, chives, and tarragon in small bowl. Measure out 1 tablespoon herb mixture and set aside for bread crumbs, then stir lemon zest into remaining herbs.

2. Heat 4 tablespoons butter in 8-inch skillet over medium heat until just melted. Add 1 teaspoon minced garlic and cook, stirring frequently, until fragrant, 1 to 2 minutes. Set skillet aside.

3. Pat fillets dry with paper towels and season both sides with salt and pepper. Turn fillets skinned side up with tail end pointing away from you. Spread 1/2 teaspoon mustard on each fillet, sprinkle each evenly with about 1 tablespoon herb--lemon zest mixture, and drizzle each with about 1 1/2 teaspoons garlic butter. Tightly roll fillets from thick end to form cylinders. Set fillets seam side down in 13 x 9-inch baking dish. Drizzle remaining garlic butter over fillets, cover baking dish with aluminum foil, and bake 25 minutes. Wipe out skillet but do not wash.

4. While fillets are baking, add remaining tablespoon butter to now-empty skillet and melt over medium heat. Add panko and cook, stirring frequently, until crumbs are deep golden brown, 5 to 8 minutes. Reduce heat to low, add remaining teaspoon minced garlic, and cook, stirring constantly, until garlic is fragrant and evenly distributed in crumbs, about 1 minute. Transfer to small bowl and stir in 1/4 teaspoon salt and pepper to taste. Let cool, then stir in reserved tablespoon herb mixture.

5. After fillets have baked 25 minutes, remove baking dish from oven. Baste fillets with melted garlic butter from baking dish, sprinkle with all but 3 tablespoons bread crumbs, and continue to bake, uncovered, until internal temperature registers about 135°F on instant-read thermometer, 6 to 10 minutes longer. Using thin metal spatula, transfer fillets to individual plates, sprinkle with remaining bread crumbs, and serve with lemon wedges.