Prepare the potatoes by peeling and chopping them into quartered pieces.
Boil the potatoes in salted water until fork-tender, about 15-20 minutes.
While the potatoes are boiling, heat the butter in a large skillet over medium heat, then add the sliced onions.
Caramelize the onions for 25-30 minutes, until they are golden and soft.
Drain the potatoes and return them to the pot over low heat.
Add the caramelized onions to the potatoes and mix in the heavy cream until smooth.
Season with salt and fresh black pepper to taste, then serve warm.