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 and toss with the olive oil, garlic, salt and pepper. Place in the oven and roast for 20 minutes, stirring after 10 minutes.
While the pepper mixture is cooking, prepare the aioli. Combine the mayonnaise, garlic, basil and lemon juice. Cover and refrigerate till needed.
Reduce the oven temperature to 350ºF. Brush the Halibut with olive oil and salt to taste. Sprinkle with the paprika. Give the peppers a stir and place the halibut on top of the peppers. Bake for 15 to 18 minutes until the halibut is just cooked through or until the flesh just turns opaque and begins to flake. Adjust the cooking time for thinner or thicker steaks.
Plate the pepper mixture and halibut, sprinkle with lemon zest, and serve with the basil aioli.