Grease a 13 x 9 inch casserole dish or other casserole dish with the same volume
Wash then peel the potatoes. Slice thinly.
Make a layer on the bottom of the casserole dish with ⅓ of the potatoes.
Sprinkle with pepper.
Cover with ½ the onions
Cover that with ½ cup of cheese
Make another layer of potatoes and repeat, ending with a third layer of potatoes
Bechamel sauce
Melt butter in a heavy saucepan over medium-low heat
Mix in the flour and salt and whisk constantly for about a minute and a half. Be careful not to scorch the bottom.
Pour hot milk into the pan slowly, whisking to keep it smooth.
Cook until the mixture thickens and coats the back of a spoon.
Add 1 cup of the remaining cheese (you should still have the ½ cup you held back for the top).
Stir until smooth
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.
Cover the dish with aluminum foil and bake for 1-½ hours
During the last 10 minutes of cooking uncover the dish and sprinkle the top with the ½ cup reserved cheese and the Panko bread crumbs.
Continue baking uncovered for 10 minutes, or until the cheese is melted and the Panko crumbs are crispy.
Let stand about 5 minutes before serving.
Notes
Expert Tip: When making a cream sauce always cook the flour for at least a minute or two to get the raw, starchy taste out.
Be sure to preheat your oven before putting this dish in the oven. This will help ensure that your potatoes bake evenly.
Make sure you slice the potatoes thinly and evenly. A mandolin or food processr can help you get those even slices.
If you want a golden brown crust on your potatoes, put them under the broiler for a few minutes after they are done baking. Just be sure to keep an eye on them so they don't burn!
Evaporated milk is what makes this dish extra creamy, using milk will take away some of that creaminess.
If your like your sauce a little on the thinner side, add a small amount of chicken broth to your bechamel sauce.
Add in a tablespoon of thyme for a flavorful twist.