Sattie, I think you could make the recipe as directed and bake it with your roast, uncovered, for at least an hour. I'd test the potatoes with a skewer or point of a knife during the cooking time to see how the potatoes are doing. I've never done scalloped potatoes in the microwave. Always in the oven.
"As a girl I had zero interest in the stove." - Julia Child
This is real inspiration. Look what Julia became!