Thaw fish, if frozen. Rinse fish; pat dry with paper towels. Using a sharp knife, make shallow bias cuts in the fish fillets, spacing cuts 3/4 inch apart. Sprinkle herbs over fillets, tucking into cuts.

Place a steamer insert in a large deep saucepan or large skillet with a tight-fitting lid. Add water to the saucepan or skillet until just below the steamer insert. Bring water to boiling. Place carrot and squash in the steamer basket. Place fish on top of vegetables. Arrange lemon slices on top of fish. Cover and steam over medium heat for 6 to 8 minutes or until fish flakes easily when tested with a fork, adding more water as needed to maintain steam.

Meanwhile, in a small bowl, stir together mayonnaise, green onion, lemon peel, and lemon juice.

To serve, divide fish and vegetables among two serving plates. Serve with mayonnaise mixture.