Begin by heating butter in a skillet over medium-low heat. Once the butter is melted, add the sliced sweet onions along with a generous pinch of kosher salt. Stir frequently to ensure even cooking and prevent any burning. You want those onions to become soft and translucent, which should take about 10 minutes.
After the initial cooking time, reduce the heat to low. This is when the magic happens! Continue to cook the onions, stirring occasionally for the next 45 to 60 minutes. Keep an eye on them; you want them to become golden and caramelized. The longer you cook them, the sweeter they will become, transforming into a lovely jam-like consistency.
Now, while the onions are caramelizing, let’s prepare the potatoes. Wash, peel, and chop the russet or yukon gold potatoes into even-sized chunks. This ensures they cook evenly. Place the chunks in a large bowl and cover them with cold water to prevent browning. You can store them in the fridge for up to 18 hours if you like, which saves time later.
When you’re ready, drain the potatoes and place them in a large pot filled with cold water. Season the water with a generous pinch of kosher salt and bring it to a boil. Boil until the potatoes are fork-tender, which should take about 12 to 15 minutes, depending on the size of your chunks. If your pieces are small, start checking around the 10-minute mark.
While the potatoes are boiling, in another saucepan, combine butter and heavy whipping cream. Heat it over medium-low until it's warmed through. This mixture will add that creamy richness we crave in mashed potatoes.
Drain the potatoes well and return them to the pot. Using a potato masher or ricer, start mashing them to your desired consistency. Don’t overdo it! We want them creamy, not gummy. Add in half of the warmed butter and cream mixture along with salt to taste.
If you prefer an extra smooth texture, now’s the time to use a hand mixer on low speed. Move it around constantly for just a minute or two to break up any larger chunks. Next, stir in the remaining butter and cream mixture and then fold in the gruyere cheese. Allow the cheese to melt gently into the potatoes, resulting in a beautifully creamy dish.
Finally, stir in half of the caramelized onions into the mashed potatoes, and taste for seasoning. This is your opportunity to adjust the salt and maybe add a touch of freshly ground black pepper, depending on your preference.
To serve, scoop the mashed potatoes into a serving dish and top with the remaining caramelized onions. You can add a sprinkle of fresh herbs for an extra pop of color and flavor if you’d like. Enjoy the rich and savory notes of your French Onion Mashed Potatoes!