This was really good. I added half of an onion, chopped, to the potatoes. I would add some chopped red pepper, parsley or something else to give a little color. I made half the recipe, and this was two good sized servings. My potatoes were not golden brown at 20 minutes, and because of schedules, I used them as is, as they were tender. I used far less salt - a few grinds of salt for the potatoes and another grind for the sauce.That bit of mustard really added flavor.
I suggest that you not start the sauce until the potatoes are nearly done - they can sit around a few minutes better than can the sauce. I used the full amount of the cheeses - the fresh mozzarella in part because I used a 6x6 baking dish. It took longer to bake than I thought. So I will make this again some morning when I don't have a timetable to meet.