Adjust oven rack to lowest position and heat oven to 500°F. Mash butter, Old Bay, and lemon juice together in a bowl; set aside. Pat halibut dry with paper towels and season with salt and pepper; refrigerate until needed.

Brush rimmed baking sheet with 1 tablespoon oil. Toss potatoes with 2 tablespoons oil, ¼ teaspoon salt, and ¼ teaspoon pepper in a bowl. Arrange potatoes, cut side down, on half of the sheet. Toss corn in now-empty bowl with remaining 1 tablespoon oil, ¼ teaspoon salt, and ¼ teaspoon pepper, then place on empty side of sheet. Nestle andouille onto sheet around corn. Roast until potatoes and andouille are lightly browned and corn kernels are plump, 20–25 minutes, rotating halfway through roasting.

Remove sheet from oven and reduce oven temperature to 425°F. Transfer corn to clean bowl, leaving andouille and potatoes on sheet. Add 2 tablespoons Old Bay butter to corn, toss to coat, and cover bowl tightly with aluminum foil; set aside.

Slide andouille to side of sheet with potatoes, then place halibut on now-empty side of sheet. Roast potatoes, andouille, and halibut until fish flakes apart when gently prodded with paring knife and registers 140°F, 8–10 minutes, rotating sheet halfway through roasting.

Remove sheet from oven. Transfer potatoes, andouille, and hailbut browned side up to serving dish. Dot remaining Old Bay butter over halibut. Add corn to serving dish, sprinkle with parsley, and serve.