There is an obvious solution to the problem that I described in this post from July. If you put cheese with the sliced potatoes in a creamy gratin, I wrote, the acidity in the cheese delays the softening of the potatoes. The solution is to add the cheese later.
The other night, I put sliced potatoes, pancetta cubes, and chopped garlic into a buttered gratin dish, with a little salt. (The dish would have benefited from some nutmeg). I poured over about 150ml of double cream (there were enough potatoes for two), stirred everything about, covered the dish with foil, and put it into a gas mark 5/190C oven.
The potatoes were tender after about 50 minutes. I gently stirred through about 110g of grated Gruyere, covered the dish again, and put it back into the oven, turned down to gas mark S/130C -- I needed only to melt the cheese. I gave it another five minutes.