Preheat oven to 425°.
Bring 1 cup water to a boil in a medium saucepan; gradually stir in couscous. Remove from heat; cover and let stand 5 minutes. Fluff with a fork. Place couscous, tomatoes, and the next 6 ingredients (through garlic) in a large bowl. Add 1 tablespoon oil, 1/4 teaspoon pepper, and 1/4 teaspoon salt to bowl; toss to combine. Set aside.
Cut 4 (15 x 24-inch) pieces of parchment paper. Fold in half crosswise. Draw a large heart half on each piece, with the fold of the paper being the center of the heart. Cut out the heart, and open. Sprinkle both sides of fillets with remaining 1/2 teaspoon salt and remaining 1 teaspoon pepper. Place 1 fillet near fold of each piece of parchment. Top each fillet with 3 lemon slices; drizzle 1 teaspoon oil over each serving. Starting at the top of the heart, fold edges of parchment, sealing edges with narrow folds. Twist the end tip to secure tightly. Place packets on a baking sheet. Bake at 425° for 12 minutes. Place on plates; cut open. Serve immediately with couscous.