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.
Ingredient notes: Bottled clam juice can be very high in sodium. We like Bar Harbor brand, which has 120 mg sodium per 2-ounce serving. Look for it in the canned-fish section or the seafood department of your supermarket.
Pacific cod, a.k.a. Alaska cod, is sustainably fished and has a larger, more stable population, according to Monterey Bay Aquarium Seafood Watch (seafoodwatch.org).
We like Ian’s brand of coarse dry whole-wheat breadcrumbs, labeled “Panko breadcrumbs.” Find them in the natural-foods section of large supermarkets. To make your own, trim crusts from firm sandwich bread. Tear the bread into pieces and process in a food processor until coarse crumbs form. (To make fine dry breadcrumbs, process until very fine.) Spread on a baking sheet and bake at 250°F until dry, about 10 to 15 minutes. One slice of bread makes about 1/3 cup dry breadcrumbs.
Tip: To peel shrimp, grasp the legs and hold onto the tail while you twist off the shell. To devein shrimp, use a paring knife to make a slit along the length of the shrimp. Under running water, remove the dark tract with the knife tip.
To Make Ahead: Bake the casserole, let cool for 1 hour, cover and refrigerate for up to 1 day; let stand at room temperature for 30 minutes, then reheat at 400°F for 40 minutes.
