Preheat oven to 425 degrees Fahrenheit and slice halibut into individual servings. Pat dry on both sides and season with kosher salt and black pepper.
Mix together the mayo, butter, garlic, green onion and lemon juice and spread evenly on top of the halibut fillets. Combine the parmesan and panko in a small bowl, and sprinkle evenly on top of the mayo mixture so it adheres to the fish.
Bake at 425℉ for about 15 minutes, or until just cooked through. To get the panko even more golden brown, turn the oven broiler to high heat and broil for about 1 minute, or until the topping is crisped.
Notes
For extra spice, add 1/4 teaspoon of red pepper flakes or sriracha to the mayo mixture.
Gluten free - Use gluten free panko like Aleia's or Jeff's Naturals.
Dairy free - Use my air fryer panko halibut recipe instead, but bake it according to these directions.
Migraine diet - Omit the lemon juice, and use mozzarella in place of the parmesan (shred it first).