One of the easy aspects of cooking gratin dauphinois is that oven temperatures ranging from gas mark "S"/130 C to gas mark 5/190 C will work. One of the difficult aspects is that you want the cream to have reduced and thickened to exactly the right consistency at the time when you are ready to eat.
At the weekend, I placed the gratin above a slowly roasting shoulder of lamb for two hours. After that time, I had planned to let the lamb rest for 30 minutes, allowing me to turn up the oven if the liquid in the gratin needed further reduction. But it did not. I covered it in foil, and put it in the lamb's place on the oven floor.
With a clear oven, I usually cook the gratin for about 90 minutes at gas mark 3/160 C.
12 charlotte potatoes (small to medium ones)
284 ml double cream
1/2 clove garlic, chopped
Peel the potatoes (waxy varieties such as charlotte work best, I think), slice them thinly, and place them in the cream in a saucepan of the same diameter as your gratin dish. Add milk, if necessary, so that the potatoes are just submerged. Add the garlic, nutmeg, and a little salt; over a gentle flame, bring the contents of the pan to a simmer. Tip into a buttered gratin dish, arranging the potatoes and pushing them down to cover them in liquid.
Bake at gas mark 3/160 C for about 90 minutes, until the potatoes are sitting in thickened, wobbly cream.