Step 1Heat the oven to 375 degrees. In a medium saute pan, heat the olive oil over medium heat. Add the onion and cook just until it's translucent, 3 to 5 minutes, stirring frequently. Season with a pinch each of salt and pepper. Set aside to cool completely.
Step 2In a small bowl, combine the mayonnaise, lemon juice, Worcestershire sauce, cheese and a few drops of Tabasco. Mix by hand until smooth. Season with one-fourth teaspoon salt and a pinch of pepper to taste.
Step 3Butter the bottom of a 12-inch by 8-inch casserole and cover evenly with the onion. Season the halibut with one-fourth teaspoon salt and a pinch of pepper and cut into 8 pieces. Place the halibut fillets on top of the onion and spread with the seasoned mayonnaise.
Step 4In a medium bowl, mix together the panko bread crumbs, butter and parsley, and season with one-eighth teaspoon salt and a pinch of pepper. Sprinkle the topping over the mayonnaise. Put the casserole in the oven and bake until the topping is golden and bubbles slightly, about 20 minutes. Serve immediately.