Preheat oven to 425 degrees. Pulse scallions, cilantro, mint, oil, ginger, coriander, and 1/2 teaspoon salt in a food processor until a coarse paste forms. Season with pepper.
Toss zucchini with 3 tablespoons herb paste in a bowl. Spread onto a rimmed baking sheet. Roast for 5 minutes.
Rub remaining herb paste onto both sides of fish fillets. Push zucchini to edges of baking sheet, and arrange fish in center, leaving about 1/2 inch between each fillet. Roast until fish is opaque and semi-firm to the touch, about 15 minutes. Meanwhile, prepare couscous according to package directions. Serve fish and zucchini over couscous.