Seafood Chowder Casserole

Name: Seafood Chowder Casserole
Ingredients: 1 tablespoon plus 1 teaspoon canola oil, divided; 2 cups sliced leeks, rinsed (about 2 small); 1 cup sliced celery; 2 large white potatoes (about 1 3/4 pounds), peeled and cut into 3/4-inch pieces; 2 cups seafood stock or clam juice (see Tips); 2 teaspoons Old Bay seasoning, divided; ½ teaspoon freshly ground pepper; 1 cup low-fat milk; ¼ cup all-purpose flour; 1 tablespoon Dijon mustard; 1 pound raw shrimp (21-25 count), peeled, deveined and chopped (about 2 cups; see Tips); 12 ounces diced cod (see Tips) or other firm white fish; 8 ounces pasteurized crabmeat, preferably jumbo, drained; 2 cups shredded Gruyère cheese, divided; 2 tablespoons chopped fresh dill or 2 teaspoons dried, divided; 1/2 cup coarse whole-wheat breadcrumbs (see Tips)