To get bigger, bolder flavor from mashed potatoes, we usually try mix-ins—halfhearted attempts to stir in seasonings, possibly cheese or buttermilk.
It usually falls flat. Buttermilk adds acidity, but little depth. And cheese too often turns stringy, lending fat but not much flavor.
Herbs and spices are a whole other problem. That's because the flavor compounds in most are fat soluble, meaning the best way to draw out the flavors of herbs and spices is combining them with a fat.
Despite all the milk and butter we often add to mashed potatoes, the dish remains mostly starch—not a good medium for fat-soluble flavor compounds.
We found our fix with an Indian flavoring technique called tarka, which loosely translates to something like “tempering.” In tarka, flavorful spices and herbs, often in the form of seeds or chilies but sometimes including garlic and ginger, are added to hot oil or butter. The temperature makes them pop with heat and quickly blooms their flavors into the fat. The flavorings are often strained from the butter, leaving behind a liquid that retains the aroma and taste of the seasonings.
For our take on tarka, we infused melted butter with a blend of yellow mustard seeds and caraway seeds, creating a peppery, complex flavor. This was delicious poured over our finished potatoes.
But because we like layers of flavor, we couldn't resist doctoring the potatoes themselves a bit. For that to work, we needed an ingredient with water soluble flavor compounds (mashed potatoes contain plenty of water). We settled on a bit of prepared horseradish and the liquid it is packed in. This added a sharpness that brightened the dish without competing with the tarka.