Bring a pot of salted water to a boil. Add the potatoes and cook until they become soft — a fork should slide through easily, about 15 minutes (depending on the size). Drain the potatoes. Set aside and let them cool.

Add the butter to a large saute pan set over medium heat. Let the butter melt, then add the thinly sliced onions and ½ tablespoon of the salt. Saute the onions, around 7 minutes, until beginning to brown and caramelize.

Add the mushrooms and potatoes and another ½ tablespoon of salt, letting them cook, stirring occasionally, for about 15-20 minutes, until the mushrooms caramelize and the potatoes begin to crisp.

In a large bowl, whisk the eggs together, and season with the remaining salt and some freshly ground black pepper. Lower the heat and add the eggs to the pan, stirring gently with a spatula to incorporate all the ingredients together. Cook until lightly set, around 4 minutes. Remove from the heat.

Season with more salt if needed and with freshly ground black pepper. Divide and plate. Serve hot, with a dollop of sour cream.