May's New and Improved Gratin Instructions

This recipe is the result of several of several experiments that are still on-going. I will update this as I go along. 

Ingredients
Potatoes, sliced thinly -- weighed after peeling and slicing
salt

Place sliced potatoes in a pot. Cover with water, add salt, and bring to a boil. Turn off the heat, and drain potatoes right away, reserving the water.

Make white sauce as per your normal recipe, starting with the milk and then following up with the reserved potato water. 

Make gratin as per usual.

Notes: 

I used water only this time, as it was an experiment. If you usually make a 100% milk white sauce, you can use the milk portion of the recipe to cook the potatoes.

There will be starch in the water, and it will thicken up more than you think. Of course, the starchier the potato that you use, the thicker it will get. So you may want to reduce the amount of flour you use in the sauce.

Most recipes that have you boil the potatoes have you cook them for much longer, until the potatoes are almost cooked through. Even worse, they may have you cook them whole and then slice or cut them. I find that doing it that way makes it difficult to get the potatoes into the pan easily. The potatoes may be slightly sticky to handle this way, but they are almost as solid as raw.

This method also allows you to preserve more flavor, as the potato water goes back into the sauce.

It is important to drain the potatoes right away, especially if they were sliced thinly. Once the slices are cooked through, they are a pain to position in the gratin pan, and letting them sit in the water is potentially enough to cook them all the way.

I also find that there is much less slumping when cooking a gratin this way, so if you do not want a short gratin, this is an excellent way to do it.