Pre heat oven to 425°F and prepare a sheet of parchment paper on a baking tray.
Place butter, breadcrumbs, Parmesan cheese, Old Bay, and lemon zest in a pan on medium heat and toast for about 5 minutes or until bread crumbs are golden brown, stirring occasionally.
In a small bowl mix mayonnaise, Dijon mustard, lemon juice, and thyme. Brush a layer of the mixture on both sides of fish and then immediately dip the fish in breadcrumb mixture.
Place fish on prepared parchment paper and bake on the middle rack for about 10-12 minutes. Serve while warm with lemon and preferred sauce.