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Preheat oven to 400°F. Coat a 9-by-13-inch (or similar 3-quart) baking dish with cooking spray.
Heat 1 tablespoon oil in a Dutch oven over medium-high heat. Add leeks and celery and cook, stirring often, until the leeks are softened, 2 to 3 minutes. Stir in potatoes, stock (or clam juice), 1 teaspoon Old Bay and pepper. Cover and bring to a simmer over high heat. Reduce heat to medium-low and simmer, covered, until the potatoes are just tender, 6 to 8 minutes.
Whisk milk, flour and mustard in a measuring cup. Stir into the potato mixture, increase heat to medium-high and bring to a simmer, stirring constantly. Stir in shrimp and fish and return to a simmer, stirring often; cook until the seafood is just cooked through, about 3 minutes. Remove from the heat and stir in crab, 1 1/2 cups Gruyère and half the dill.
Transfer the seafood mixture to the prepared baking dish. Mix breadcrumbs with the remaining 1 teaspoon each oil and Old Bay. Stir in the remaining 1/2 cup Gruyère and the remaining dill. Sprinkle the breadcrumb mixture over the casserole.
Bake the casserole until it is bubbling and golden brown, 20 to 30 minutes. Let stand 10 minutes before serving.