The perfect bowl of mashed potatoes is creamy yet fluffy and well-seasoned throughout, but it’s not enough to add salt and seasonings after, or even during the mashing step. Infusing potatoes with flavor starts at the beginning, at the boiling step.
Just like pasta, your spuds should be cooked in salted water so every bite is evenly seasoned. Unlike pasta, you should go one step further and add aromatics and garlic to your boiling water. (I guess you could do this with pasta, too, if you really wanted to, but I think it would make the Italians angry.)
Adding herbs (usually thyme or rosemary) and smashed garlic cloves to the boiling water enhances the flavor of the potatoes without overpowering them. The herb sprigs, especially woody ones like rosemary and thyme, should be removed, but the garlic can travel with the potatoes all the way to their final destination. The cloves will be mellowed and softened by the boiling, bringing a sweet, subtle, not-so-pungent flavor to your mash. Just run it through your food mill or ricer along with the potatoes (or mash with a masher).
If you want to double up on infused flavor, you can infuse the liquid dairy component of your mash with even more herbs by adding a few sprigs to your heavy cream or half & half and gently warming the dairy over low heat. The fat will extract slightly different flavors from the aromatics (as some compounds are fat soluble while others are water soluble), giving your bowl of mashed spuds layers of flavor.
Start by stirring salt into your water, and heating as needed to dissolve. You want it to be noticeably salty, like the ocean, or the tears of your enemies. Add a couple of sprigs of thyme, or rosemary, or both, along with three or four cloves of garlic. Add the potatoes and boil as usual, until they offer no resistance when pierced with the tip of a knife. Remove the herb sprigs, then mash the potatoes (and garlic) as usual.
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