Preheat the oven to 500 degrees, with the rack in the lowest position.
In a bowl, combine butter, Old Bay, onion powder, and lemon juice, set aside
Pat halibut dry, and season with salt and pepper, place on a plate and put in the refrigerator uncovered while you prepare the rest.
Take a baking sheet, and brush it with 1 tablespoon of oil.
In a large bowl toss the potatoes with 2 tablespoons oil, ¼ teaspoon salt, and ¼ teaspoon pepper. Pour onto one side of the baking sheet.
Add corn to the bowl the potatoes were in, and toss with remaining oil, and a little salt and pepper. Then pour onto the other side of the baking sheet. Add andouille sausage around it.
Place in the oven, and roast for 20-25 minutes, rotating the sheet half way through roasting time.
Remove from the oven, reduce oven temp to 425 degrees F, and use tongs to take corn off the sheet. Transfer to a clean bowl, and toss with half the seasoned butter. Cover with foil, and set aside.
Use a wooden spoon to push the andouille sausage to the side of the sheet with the potatoes, and use the empty space on your baking sheet for the halibut.
Return the tray to the oven, and continue roasting for 8-10 more minutes, until the internal temp of the fish is 140 degrees, and it flakes easily.
Remove from the oven, and place everything together on a serving dish, including the corn, use remaining butter to top the halibut, and sprinkle everything with parsley, and serve.