Preheat the oven to 400°F. Butter a 9-by-13-inch baking dish with 1/2 T of butter. Distribute the scallops and shrimp across the bottom of the dish.
Heat 1 T of butter and the olive oil in a nonstick skillet over medium heat until hot and bubbly. Sauté the pieces of cod for two minutes on each side. Transfer to the casserole, distributing the pieces among the scallops and shrimp. Season with kosher salt and fresh cracked pepper.
Add shallots to the skillet; sauté for 3 to 4 minutes to soften. Add the garlic and cook for another minute. Add the mushrooms and 1 T of butter. Season with salt + pepper; sauté for 4 to 5 minutes. Add the flour and cook for one more minute, stirring continuously. Turn the heat down to medium-low.
Whisk in the sherry, Dijon mustard and heavy cream all at once. Cook for a minute or two to thicken. Season with salt + pepper to taste. Pour the sauce over theseafood and sprinkle with parsley. (If the sauce seems thick and like it won't cover the casserole, don't worry. Spread it as evenly as possible. As the seafood bakes, it will release juices and mix with the sauce.)
Melt the remaining tablespoon of butter and combine with the cracker crumbs. Distribute over the top of the casserole. Bake for 15 to 20 minutes. (The internal temperature of the cod should read 145°F.)
