Let’s get cooking! Start by peeling and thinly slicing the onions—it might seem like a lot, but trust the process. Heat up the olive oil in a large pan over medium heat, then add all the onions. Stir occasionally and let them cook until they turn brown and caramelized, which should take about 45–50 minutes. Be patient; it’s worth it! The onions will shrink as they cook.
While the onions are caramelizing, chop the potatoes into small pieces. No need to peel them! Boil them in a pot of water for about 20 minutes until they’re fork-tender. Drain the water and transfer the potatoes back to the pot. Mash them up and add the cream, butter, and salt. Give it another mash to mix everything together.
Once the onions are beautifully caramelized, add half of them to the mashed potatoes along with the parmesan cheese. Give it all a good stir and taste to adjust seasoning if needed. Plate up the potatoes, then sprinkle fresh chives, sage, and melted butter on top. Now, it’s time to dig in and enjoy!