Stir 1 cup lemon juice 1/2 cup mint, sugar, fish sauce, 1 teaspoon lemon peel, and chili paste in small bowl until sugar dissolves. Set Thai lemon-mind sauce aside.
Mix remaining 1/2 cup lemon juice, 1 cup mint, and 2 teaspoons lemon peel in small bowl. Set mint topping aside.
Heat oil in heavy medium skillet over medium-high heat. Add lemongrass and shallots and saute until shallots are slightly softened and golden, about 6 minutes. Cool.
Place 1 banana leaf or parchment sheet on work surface. Place 2 tablespoons cold shallot-lemongrass mixture in center of banana leaf. Place 1 fish fillet on lemongrass mixture on bananaa leaf. Sprinkle fish with salt and pepper, top with 1 tablespoon butter; spoon 2 tablespoons mint topping over. Fold 1 short side of banana leaf over fish. Fold long sides inches Roll up to completely enclose fish. Repeat with remaining banana leaves, shallot-lemongrass mixture, fish, butter, and mint topping.
Preheat oven to 450°F Place fish packets on rimmed baking sheet and roast until fish is opaque in center, about 10 minutes. Unwrap fish and serve with Thai lemon- mint sauce.