A nostalgic midcentury Alaskan casserole of tender halibut baked over sweet onions with a rich, tangy cream topping and crisp buttery crumbs.
6 (5-ounce) skinless halibut fillets
2 cups dry white wine
1 1/2 tablespoons unsalted butter, plus softened butter for greasing
1 large white onion, sliced into 1/4-inch-thick rounds, and rings separated (about 3 cups)
1 1/2 teaspoons kosher salt
1/2 teaspoon black pepper
1/2 cup mayonnaise
1/2 cup sour cream
1/2 teaspoon Dijon mustard
Hot sauce, to taste (such as Tabasco)
12 round buttery crackers (such as Ritz), roughly crushed (about 2/3 cup)
Finely chopped fresh flat-leaf parsley
Place halibut in an even layer in a 13- x 9-inch baking dish. Pour wine over halibut and turn to coat. Cover and refrigerate for 2 hours, turning once halfway through.
Meanwhile, melt butter in a large skillet over medium-high. Add onion rings; cook, stirring occasionally, until soft and lightly golden, about 12 minutes. Transfer to a small heatproof bowl and set aside until ready to use.
Preheat oven to 425°F. Remove halibut from wine and pat dry with paper towels. Discard wine; wash and dry baking dish. Coat bottom of baking dish with softened butter; evenly spread cooked onions in bottom of dish. Sprinkle both sides of halibut with salt and pepper; place in single layer on top of onions. Set aside.
Stir together mayonnaise, sour cream, Dijon, and hot sauce to taste in a small bowl until combined. Spread a thick layer of mayonnaise mixture over top of each halibut fillet (about 2 1/2 tablespoons each). Sprinkle evenly with crushed crackers.
Bake until thickest piece of halibut registers 130°F and is no longer translucent in center and all fillets flake easily with a fork, 15 to 18 minutes. Garnish with parsley.
