In a large saucepan, combine potatoes, peeled and thinly sliced; leeks, garlic, salt, pepper and nutmeg.
Whisk flour into milk and stir into potatoes. Simmer 6 minutes, stirring occasionally.
Spoon into 8 x 8 x 2-inch baking dish; fold in 1/3 cup grated Parmesan cheese. Top with additional 1/3 cup Parmesan and 1/3 cup panko crumbs. Bake at 350 degrees F for 40 minutes.