Preheat the oven to 400 degrees F.
In a small nonreactive saucepan, melt the butter over low heat. Remove from the heat and whisk in the flour, Return pan to the stove and cook for 2 minute over moderate heat, stirring constantly, until the flour is cooked through. Remove from the heat, and pour in the fish stock. Return the pan to the stove and cook over moderate heat, whisking constantly for 2 to 3 minutes, or until the mixture forms a light sauce. Stir in the cream and cook over medium heat, stirring frequently, until the sauce is the thickness you prefer. (This is a light thin sauce, designed to baste the fish as it bakes.) Set the mixture aside.
Place the shrimp, scallops, and haddock in a buttered baking dish just large enough to hold them. Pour over the sauce. Bake for 8 to 10 minutes. Remove the dish from the oven, tilt, baste with the cooking juices, and add the lobster. Sprinkle with the cheese, then top with the Cracker Crumb Mixture. Return the dish to the oven and bake for 5 to 8 minutes longer, or until the crumbs are lightly browned.