Believe it, or not, I have never made scallopped potatoes...much less perunagratiini, Finnish scalloped potatoes. These were so good that we scraped the pan clean.
I layered scrubbed, but not peeled, sliced potatoes with sliced onions. Season the dish with freshly ground salt and pepper. Then fill the baking dish half way with heavy cream, cover with foil, and bake at 350 degrees for 25 minutes. Uncover and bake for another 25 minutes until the edges of the potatoes are slightly crisped.