Ham and Potato Casserole

Name: Ham and Potato Casserole
Ingredients: Cooking spray or butter, for coating the pan; 2 pounds russet potatoes (about 4 medium); 1 cup diced, cooked ham (about 5 1/2 ounces); 1/2 medium yellow onion; 8 ounces sharp cheddar cheese, shredded (about 2 cups); 2 tablespoons unsalted butter; 2 tablespoons all-purpose flour; 2 cups whole or 2% milk; 1 teaspoon kosher salt, plus more as needed; 1/8 teaspoon freshly ground black pepper