Mashed potatoes are notoriously hard to make ahead, often turning gluey or gray during reheating. Not the case with this fluffy casserole, which can sit up to 2 days in the fridge before you finish it off in your June to praise and acclaim (and fewer dishes on the day). Be sure to use gold potatoes and use a gentle hand while whipping for smooth, creamy results, and don’t hesitate to play around with the ingredients--you can use an equal amount of Boursin or goat cheese instead of cream cheese, stir in grated Gruyere or cheddar or even top with breadcrumbs for a crispier crust. The potato skins may seem dark when removed from the oven, but don't worry--the flesh will be perfectly cooked.