Creamy Potatoes au Gratin

0 Preheat oven to 400F
1 Grease a 13 x 9 inch casserole dish or other casserole dish with the same volume
16 Wash then peel the potatoes. Slice thinly.
2 Make a layer on the bottom of the casserole dish with 1/3 of the potatoes.
3 Sprinkle with pepper.
4 Cover with 1/2 the onions
5 Cover that with 1/2 cup of cheese
6 Make another layer of potatoes and repeat, ending with a third layer of potatoes
Bechamel sauce 
7 Melt butter in a heavy saucepan over medium-low heat
8 Mix in the flour and salt and whisk constantly for about a  minute and a half. Be careful not to scorch the bottom.
9 Pour hot milk into the pan slowly, whisking to keep it smooth.
10 Cook until the mixture thickens and coats the back of a spoon.
11 Add 1 cup of the remaining cheese (you should still have the 1/2 cup you held back for the top).
12 Stir until smooth
13 Carefully pour the cheese sauce over the potatoes letting it soak down to all layers. You may need to gently move the potatoes from the sides to "help it" along.
14 Cover the dish with aluminum foil and bake for 1-1/2 hours
15 During the last 10 minutes of cooking uncover the dish and sprinkle the top with the 1/2 cup reserved cheese and the Panko bread crumbs. 
18 Continue baking uncovered for 10 minutes, or until the cheese is melted and the Panko crumbs are crispy.
19 Let stand about 5 minutes before serving.