Let's consider two facts: The key to an easy Thanksgiving is to make as much of the food ahead as possible. But cold mashed potatoes don't reheat very well, and are best made at the last minute.
The logical conclusion is that if you want mashed potatoes on the Thanksgiving table, you either have to make them right before serving, or drop the idea entirely. But as anyone who's had a mashed potato casserole before knows, those aren't the only two options.
Yet even when loaded with cream and butter, mashed potatoes tend not to reheat well: they harden when chilled and, without the addition of moisture, never regain their freshly-made taste and texture when reheated. The trick, then, is to add an ingredient that will help maintain the mash's moisture level so that it softens back to its original light, fluffy texture when cooked again.
There are already a lot of recipes for this, and most call for adding cream cheese or sour cream to the mix. I had two main goals when creating this recipe: first, I wanted to make sure that those really are the best add-ins, and then I wanted to create a really flavorful crispy topping to make this casserole feel like special-occasion—not everyday—mashed potatoes. The flavors of a fully loaded baked potato with bacon, cheese, and scallions seemed like an obvious candidate, especially once you consider the tangy dairy that'll be joining the mash.
Since some mashed potato casserole recipes I found called for cream cheese and some called for sour cream, I first wanted to better understand how each of those ingredients affects the final flavor and texture of the mashed potatoes, and then test some other add-ins to make sure there wasn't a better option lurking on the sidelines.
To test it out, I made a big batch of mashed potatoes using russet potatoes, heavy cream, and butter, then split it into portions. I left one plain as a control, then mixed equal amounts of creamy add-ins into each of the others and refrigerated them overnight. The next day I reheated the potatoes and tasted my way through them. Here's what I found:
- Plain Mashed Potatoes With Butter and Cream (Control): A little bit drier and denser than the other ones, but not as much as I had expected; it has an intense potato flavor that isn't entirely pleasant, almost like instant mashed potatoes.
- Full Fat Greek Yogurt: The lactic tang of Greek yogurt overwhelms the potatoes, making them taste like spoiled milk. The potatoes are also surprisingly dry.
- Mascarpone: Loaded with butterfat, the mascarpone creates mashed potatoes that are disgustingly rich; in fact, there's so much butterfat in these that they weep grease.
- Bechamel Sauce: It had worked well for my arancini recipe, so I thought maybe it would also work here. Wrong! The bechamel, even though delicious plain, gives an unpleasant floury taste to the reheated mashed potatoes.
- Sour Cream: My personal favorite, it creates reheated mashed potatoes that are moist and light, with a pleasant tang. The flavor makes me think of baked potatoes with sour cream.
- Cream Cheese: Very good. The flavor is more mild, letting the mashed potatoes shine through, though the mash is more dense than the sour cream version.
After this test, I was convinced that sour cream and cream cheese are indeed the best ways to go. Because the sour cream produced reheated potatoes that were especially light, it's the one I use in my recipe (plus the baked-potato flavor of the sour cream inspired my crispy topping idea, which I'll explain below). But you can substitute an equal amount of cream cheese with very good results, or even do a combination of the two.
Making the Mash
Having settled on sour cream, the next step was to make the full batch of mashed potatoes. I started by using Kenji's technique of rinsing the cubed russet potatoes both before and after boiling. The goal is to remove as much of the starch on the cut surfaces of the potatoes as possible, since that starch is one of the main causes of gluey mashed potatoes.
Once the potatoes are cooked, I drained them and spread them out on baking sheets, then transferred them to a warm oven for several minutes to dry. I find this step important: by removing excess water moisture, the potatoes are able to absorb more butter and cream before becoming thin and soupy.
We only had a potato masher* in the office, so that's what I used in the photos here, but you can also rice the potatoes or pass them through a food mill. The potato masher produces mashed potatoes with some lumps; some people love those rustic chunks, some don't mind them (I'm in this camp), and some hate them. If you hate them, definitely use a ricer or food mill instead.
* Check out another cool use of a potato masher here.
Overworking the potatoes is another thing that can make them gluey, so I added the butter and sour cream to the potatoes before I started mashing them, killing two birds with one stone and reducing the total time spent working the potatoes. If you use a ricer or food mill, you will have to do it a little different: with a ricer, you'd rice the potatoes first, then mix the butter, sour cream, and cream in afterwards; if using a food mill, you can mill the butter through with the potatoes, and then mix the sour cream and cream in after.
Once the potatoes were mashed, I added the cream—adding it any earlier just leads to a lot of sloshing and mess.
It may look a little too wet at first, but once the cream is evenly mixed in, the mash should be soft but not too liquid-y.
Then I scraped the mashed potatoes into a baking dish, smoothed the surface, covered it with plastic, and refrigerated it until ready to finish the casserole for serving. In my testing, I let the pre-made mashed potatoes sit in the fridge for as long as five days, and they still reheated beautifully.
The Crispy Topping
As I mentioned above, the sour-cream flavor of the mashed potatoes got me thinking of baked potatoes with sour cream and all the classic fixings: bacon, cheddar, and scallions.
To make a casserole topping that incorporates all those ingredients, I mixed panko bread crumbs with crispy diced bacon along with its rendered fat, thinly sliced scallions, and grated cheddar.
About two hours before serving, I took the potatoes out of the refrigerator and let them stand at room temperature for about an hour, just to remove some of their extreme chill before putting them in the oven.
I distributed the topping all over the mashed potatoes and set the casserole in a 350°F oven for about 45 minutes until the potatoes were hot and the topping was browned.
The result is a mashed potato casserole that can be made well in advance, and then reheated to produce light, fluffy and moist potatoes with a subtle sour-cream tang and flavorful crunchy topping that recalls the best fully-loaded baked potatoes.