Potato, Leek, and Ham Soup

Name: Potato, Leek, and Ham Soup
Cuisine: American
Ingredients: 4 cups milk; 1 ham bone or ham knuckle with ham; 1 ham bone or ham knuckle with ham; 3 cups diced potatoes (use a combination of Idaho, Maine, and new-red potatoes); 3 cups diced potatoes (use a combination of Idaho, Maine, and new-red potatoes); 3 cups diced potatoes (use a combination of Idaho, Maine, and new-red potatoes); 3 cups diced potatoes (use a combination of Idaho, Maine, and new-red potatoes); ¼ pound butter or ½ cup bacon fat; ¼ pound butter or ½ cup bacon fat; 4 leeks—washed, split, and the white part cut into 2-inch julienne; 4 leeks—washed, split, and the white part cut into 2-inch julienne; 1 cup chopped celery and celery leaves; 1 cup chopped celery and celery leaves; 2 tablespoons mustard seed, crushed; Salt and freshly milled black pepper; Salt and freshly milled black pepper; 2 tablespoons all-purpose flour mixed with 2 tablespoons softened unsalted butter; 2 tablespoons all-purpose flour mixed with 2 tablespoons softened unsalted butter