One of my favorite ways to cook halibut when it's in season! This is simple and tasty--I someimes slice a lemon and roast the slices along with the fish, but it's not really necessary. As with all fish dishes, the fresher the fish, the better.
Preheat oven to 450 degrees. Put sliced potatoes, olive oil, rosemary, and garlic in a large (12" or so) cast iron pan and mix well. Roast for 15-20 minutes, until potatoes are almost tender.
Lay halibut fillet on top of potatoes, sprinkle with chopped rosemary, chopped parsley, lemon juice and zest. Bake 8-10 minutes per inch of halibut thickness, or until just cooked through.