Preheat oven to 350°.
Cut green onion into 3-inch pieces, and cut pieces into julienne strips.
Heat oil in a large nonstick skillet over medium-high heat. Add garlic; sauté 30 seconds or until garlic begins to brown. Add the tomato and onions; reduce heat to medium, and cook for 1 minute. Remove from heat; stir in 2 tablespoons lemon juice, vinegar, and 1/2 teaspoon salt. Keep warm.
Combine 1 tablespoon lemon juice, 1/4 teaspoon salt, and broth in a medium saucepan; bring to a boil. Gradually stir in couscous and chopped chives. Remove from heat; cover and let stand 5 minutes. Fluff with a fork. Cover and keep warm.
Sprinkle fish with 1/4 teaspoon salt and pepper. Place fillets in an 11 x 7-inch baking dish coated with cooking spray. Place 4 halved lemon slices on each fillet. Bake at 350° for 20 minutes or until fish flakes easily when tested with a fork. Serve over couscous, and top with vinaigrette. Garnish with whole chives, if desired.