The inclusion of caramelized onions takes these mashed potatoes over the top: they have the depth of flavor and sweetness particular to slow-cooked onions, but the lightness and fluffiness of our favorite mashed potatoes. Pureeing the onions means that every bite of potato gets some of that oniony richness; a hand blender, like the Braun MultiQuick 9, does the trick in a second. —The Editors
grated Parmigiano Reggiano, or more to taste
Milk or cream, as needed
Salt and pepper, to taste
Over medium-low heat, melt a few tablespoons of butter in a thick-bottomed stainless steel or cast iron pan. Add the sliced onions and cook for 10 minutes to soften. Add a pinch of salt, then reduce the heat and continue to cook the onions for 30 minutes to one hour until they are deep amber in color and caramelized. Stir them often enough so that they don't burn, but not so much that they won't brown.
While the onions are cooking, put the potatoes in a stock pot filled with water. Generously salt the water, bring to a boil, and cook the potatoes until they are tender. Drain the potatoes, let them cool until you can handle them, and then peel.
Purée the caramelized onions in a food processor with the remaining butter until you have a smooth paste. Then mash this purée into the potatoes, along with the grated Parmigiano Reggiano. If you like smooth, silky mashed potatoes, use a hand blender. If you like it chunkier, use a potato masher or the back of a wooden spoon. If your mash looks dry, add milk or cream in small increments until you've reached the desired consistency. Season with salt and pepper to taste.