Start by peeling the russet potatoes. Once peeled, slice them into uniform ¼-inch thick rounds for even cooking.
Soak the sliced potatoes in a bowl of cold water for about 30 minutes to remove excess starch and prevent browning. Drain and pat dry with a clean kitchen towel.
In a large skillet, melt 2 tablespoons of unsalted butter and 2 tablespoons of extra virgin olive oil over medium heat. Add the thinly sliced onions and a pinch of salt. Cook for 10-15 minutes, stirring occasionally, until caramelized and golden brown. Remove onions and set aside.
In the same skillet, add the remaining 2 tablespoons of butter. Once melted, add the sliced potatoes in an even layer. Season with salt and pepper. Cook without stirring for 5-7 minutes until the bottom side begins to brown.
Carefully flip the potatoes using a spatula to brown the other side. Add the caramelized onions back into the skillet and gently mix. Cook for another 10-15 minutes, stirring occasionally, until potatoes are tender and golden brown.
Remove the skillet from heat. Sprinkle chopped fresh parsley over the potatoes. Serve warm and enjoy!