Preheat oven to 450°F. Lightly grease a 13×9-inch baking dish.
Rinse the halibut portions and pat dry with paper towels. Arrange the 6 portions in a single layer in the prepared baking dish.
Sprinkle the halibut evenly with 1/4 teaspoon salt.
In a small bowl, combine 1/4 cup shredded smoked Swiss cheese, 1/4 cup low fat sour cream, 1/4 cup fat-free plain yogurt and 2 tablespoons thinly sliced green onions; stir until evenly mixed.
Spread the cheese–sour cream mixture evenly over the top of each halibut portion.
Sprinkle 1/8 cup grated Parmesan cheese evenly over the coated halibut.
Bake, uncovered, for 12 to 15 minutes, or until the halibut flakes easily with a fork.
Remove from oven and sprinkle with the remaining 1 tablespoon thinly sliced green onion before serving.