Humble Ham Casserole

Name: Humble Ham Casserole
Ingredients: 3 tablespoons unsalted butter, plus more for the baking dish; 2 medium leeks (white and light green parts only), thinly sliced; 1 tablespoon chopped fresh thyme; 1/4 cup all-purpose flour; 2 cups milk; 1 cup half-and-half; Pinch of freshly grated nutmeg; Kosher salt and freshly ground pepper; 3 medium russet potatoes (about 2 pounds); 2 cups shredded medium-sharp cheddar cheese (about 8 ounces); 2 cups diced baked ham; 2 tablespoons breadcrumbs; 1/4 cup grated parmesan cheese