Step 1

Preheat oven to 400 degrees F. Coat a 9-by-13-inch (or similar 3-quart) baking dish with cooking spray.

Step 2

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.

Step 3

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 Gruyere and half the dill.

Step 4

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 Gruyere and the remaining dill. Sprinkle the breadcrumb mixture over the casserole.

Step 5

Bake the casserole until it is bubbling and golden brown, 20 to 30 minutes. Let stand 10 minutes before serving.