Mashed Potatoes With Caramelized Onions

• 4 to 6 medium-large Russet or Yukon Gold potatoes
• 10 tablespoons unsalted butter, divided
• 2 large yellow onions, halved and thinly sliced
• 1/4 cup grated Parmigiano Reggiano, or more to taste
• 1 splash Milk or cream, as needed
• 1 dash Salt and pepper, to taste

1. 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.
2. 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.
3. Puree the caramelized onions in a food processor with the remaining butter until you have a smooth paste. Then mash this puree 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.

potatoes, unsalted butter, yellow onions, milk, salt

Taken from food52.com/recipes/31816-mashed-potatoes-with-caramelized-onions (may not work)