Start by making the marinade/topping for the fish by whisking together all of the ingredients for the halibut, minus the fish.
Coat the fish with the mixture and set aside in the refrigerator to marinate while you begin preparing the vegetables.
Preheat oven to 400°.
Place the vegetables in a bowl. Drizzle with olive oil. Add the rest of the ingredients for the vegetables and toss to coat.
Line a baking sheet with parchment paper, aluminum foil, or a silicone baking mat. Coat with nonstick cooking spray if you use aluminum foil. Transfer the vegetables to the baking sheet, leaving a bit of room for the fish in the middle. Bake for 5 minutes.
Remove the baking sheet from the oven. Place the fish in the middle between the vegetables. Bake for about 25 minutes or until the potatoes are cooked through and the fish is flaky.