Scalloped Potatoes

A problem with a slow oven roast beef recipe is that it makes it hard to get anything else in the oven. If you don't have a double oven, how can you cook anything else? I tried the following.

Eight medium potatoes, sliced thin in a mandoline. Butter a 9 x 12 baking pan, and lay down a layer of potatoes. Salt and pepper, and sprinkle in a thin layer of shredded swiss or gruyere cheese, then another layer of potatoes, etc, going on to three layers of potatoes, ending with cheese on top. Heat two cups of cream and half a stick of butter for four minutes in the nuke, and pour over the potatoes.

Next, it ought to say, place in a 350 oven and bake for an hour, but when can we do that? I put it in the oven with the beef for the 45 minute last cooking period, but it wasn't close when the beef came out, so I cranked the oven to 425 and waited.... truth it it wasn't really done when it was dinner time. Good, but not quite done. Next time, maybe I'll try precooking them with the beef, and then finishing them in the 300 oven later, though that's a lot of cream to be sitting around warm all aftenoon. Maybe I'll just get that double oven.