Preheat oven to 400°. Place each onion slice on a double thickness of heavy-duty foil (about 12 in. square). Top with salmon. Combine next seven ingredients; spoon over fillets. Fold foil around mixture; crimp edges to seal.
Place on a baking sheet. Bake until fish flakes easily with a fork, 15-20 minutes. Be careful of escaping steam when opening packets. Serve with mint and lime zest.