In Romania, mămăligă cu brânză și smântână, or polenta with cheese and sour cream, can be the center of a meal or served as a side to roasts and braises. We prefer the robust texture and flavor notes of coarse polenta or stone-ground yellow cornmeal. Instant polenta can be ready in a flash, but its taste and consistency are less nuanced. If you wish to use instant polenta, cook 1½ cups and 1 tablespoon salt in 6 cups water according to the instructions on the package. Add the butter and pepper to the finished polenta as directed. The dish can be made ahead. Assemble the layers in the baking dish, but stop short of adding the sour cream and final sprinkling of cheese. Cover and refrigerate for up to 24 hours. When ready to bake, uncover and top with the sour cream and cheese. Cover with foil, bake at 425°F for 30 minutes, or until warm at the center, then uncover and bake until spottily browned, another 20 to 25 minutes.