Thaw fish, if frozen. Rinse fish; pat dry with paper towels. In a large skillet or saucepan place a large open steamer basket over 1/2 inch of water. Bring water to boiling; reduce heat. Carefully place the fish fillets in the steamer basket. (If necessary, cut the fish into 2 pieces to fit.) Top with half of the lemon slices. Cover and steam fish about 6 minutes or until fish flakes easily when tested with a fork. Discard lemon slices.
Remove fish from steamer basket. Carefully immerse the fish in a bowl of ice water. Let the fish stand in the ice water about 5 minutes or until thoroughly chilled. Remove fish from water; drain on paper towels. (Or, cover and refrigerate for 2 to 4 hours or until chilled.)
Meanwhile, for sauce, cut up any large pieces of cucumber or mixed vegetable salad. Stir together the salad and the salsa.
Arrange mixed greens on 2 dinner plates. Place the chilled fish on top of the greens. Spoon salsa mixture over fish. Garnish with remaining lemon slices. Makes 2 servings.