Creamy Seafood Chowder Casserole

Name: Creamy Seafood Chowder Casserole
Ingredients: 2 medium Yukon Gold potatoes (divided, thinly sliced with a mandoline (about 1/8 inch thick)); 1/2 teaspoon salt (divided); 1/4 teaspoon ground black pepper (divided); 2 tablespoons butter (to saute the onion and garlic); 1 small onion (finely chopped); 2 cloves garlic (minced); 3 tablespoons butter (to make the sauce); 3 tablespoons all-purpose flour; 1 1/2 cups milk (preferrably full fat); 1 cup chicken broth (or seafood broth); 1/2 cup heavy cream; 1 cup mozzarella cheese (grated - or white cheddar, or a mix); 1/2 teaspoon salt; 1/4 teaspoon black pepper; 1 teaspoon Old Bay seasoning; 1 teaspoon Dijon mustard (optional but adds depth); 12 oz. (350 gm.) salmon fillet (skin removed, cut into bite-size cubes); 12 oz. (350 gm.) cod or halibut fillet ( cut into bite-size cubes); 1 cup cooked crab (lump or imitation, chopped); 1 cup bay scallops (or sea scallops, chopped); 1/2 cup smoked salmon (chopped into small pieces); 3/4 cup breadcrumbs (panko or regular); 2 tablespoons butter (melted); 1 tablespoon scallion (chopped - or fresh parsley, or dill)