Advice on time for cooking potatoes au gratin? So, I was following a recipe in an Amish cookbook that was pretty minimal on details:


  Baked Pork Chops
  
  Put a layer of thinly sliced potatoes
  into a baking dish, and cover them
  with shredded onions, salt and pepper.
  Lay over this as many pork chops as
  needed, seasoned well on both sides. 
  Add enough milk to moisten the
  ingredients, then bake them at 400°
  until done.


(note, the rest of the most of the other recipes have ingredient amounts, baking time, etc ... but this is kinda like one of those assembled church / military wives club / etc cookbooks where there's lots of different recipes and some are of a different style)

Now, I admit, I didn't realize until after the pork was cooked that I had set the oven to 350°F and not 400°F ... but as I had remembered problems in the past with cooking potato au gratin before, and it taking a really long time, I opted to use two dishes, so the layered potatoes weren't more than 2 cm (~3/4 inch) thick, and the second dish was au gratin, without the pork, and with cheese added between layers.  (I broke out the mandolin, and had maybe 1mm thick slices).

Now, it might've been that because I cut the potatoes so thin that testing for doneness with a knife just didn't work ... so I went and looked up recipes online to see if I could find a pattern between time/temperature/thickness of the potato slices/thickness of the assembled layer ... and I'm at a loss, as there are a lot of recipes that don't even mention what size baking dish to use, so they'd end up with different layered thicknesses, and some don't mention the thicknes of the slices to use.

Is there some formula to use?  If there isn't, and I just have to go by sticking a knife in it 'til it comes out easily (or is that a bad test?), are there any tips so it doesn't take almost 2 hrs to cook?
Microwave the potatoes before putting them in the oven for about 7 minutes(start with 5 minutes, add 30 seconds every time till done), covered with a little water (very little, couple of spoons full)(potatoes whole or sliced)

Make sure you're not using a 'heavy' oven dish, glass ones can taka a long time to heat up. Place the oven dish into the oven while heating and add a drop of oil. When you put in the potatoes you should hear a little 'hiss' indicating the termperature is right.