Advice on time for cooking potatoes au gratin?
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.