Heat the oven to 400ºF.
Cut peppers into ½-inch strips lengthwise. Halve the onion and cut into ¼-inch slices lengthwise. Place both on a 13 x 18-inch rimmed baking sheet. Put the olive oil, garlic, salt and pepper in a small bowl. Mix well and brush on both sides of the peppers and onions. Place in the oven and roast for 20 minutes, flipping after 10 minutes. When done, set aside and keep warm in aluminum foil. You can reuse the same rinsed out sheet to bake halibut.
While the pepper mixture is cooking, prepare the aioli. Combine the mayonnaise, garlic, basil, lemon juice, cayenne, salt, and pepper. Mix well. Cover and refrigerate till needed.
Reduce the oven temperature to 350ºF. Brush the halibut with olive oil, salt, and pepper to taste. Sprinkle with the paprika. Bake for 15 to 20 minutes depending on thickness of the steaks.
Plate the pepper mixture and place halibut on top. Sprinkle generously with lemon zest, and serve with the basil aioli.
