Place the caramelized onions on a baking dish and spread evenly. Preheat oven to 375F.
1 cup caramelized onions
Season the halibut with salt on both sides. Place the halibut on top of the caramelized onions.
1 lb halibut, ½ teaspoon kosher salt
In a bowl combine the sour cream, mayo, minced garlic, lemon juice and dijon mustard. Stir to combine and add half of the parsley.
¼ cup mayonnaise, ¼ cup sour cream, 1 clove garlic, 1 tablespoon lemon juice, 1 tablespoon parsley, 1 teaspoon dijon mustard
Smear the mayo mixture on top of the halibut.
In another bowl combine the panko, Parmesan and the rest of the parsley.
⅓ cup panko bread crumbs, ⅓ cup Parmesan cheese
Top the halibut with the panko mix. Bake at 375F for 20 minutes.
Broil on low for 2 minutes to make the panko crumbs golden brown, keep an eye on it as broiling can burn things easily.