Begin by peeling the russet potatoes. Once peeled, slice them into uniform rounds, about 1/4 inch thick. Place the sliced potatoes in a bowl of cold water to prevent browning.
In your large skillet, heat 2 tablespoons of unsalted butter and 2 tablespoons of extra virgin olive oil over medium heat. Add the thinly sliced onion and a pinch of salt. Sauté for about 10-15 minutes until golden brown.
Drain the potatoes and pat them dry. In the same skillet, add the remaining 2 tablespoons of unsalted butter. Once melted, add the potato slices in a single layer, seasoning with salt and pepper.
Increase the heat to medium-high and cook the potatoes for about 5-7 minutes without stirring. Carefully flip the potatoes and cook for an additional 5-7 minutes until golden and crispy.
Once crispy, return the caramelized onions to the skillet. Gently toss to combine, remove from heat, sprinkle with parsley, and adjust seasoning as needed.