Peel and slice potatoes into 1/4 inch thick rounds.
Place the potato rounds in a stock pot and cover with cold water. Season with salt.
Bring the water to a boil then turn to low and simmer the potatoes for 4 minutes.
Drain the potatoes and set aside to cool.
Melt the butter in a large skillet over medium heat. Add the onions to the pan and cook, stirring occasionally, until caramelized, approximately 10-12 minutes. Season with salt and pepper.
Transfer the onions to a bowl and set aside.
Melt the remaining butter in the skillet and add the potato rounds. Season with salt and pepper.
Toss the potatoes to coat with the butter and then arrange in an even layer in the skillet.
Cook the potatoes until the bottoms are golden brown and crispy, approximately 4-5 minutes. Turn the potatoes and cook for an additional 6-7 minutes or until the potatoes are golden brown.
Return the onions to the pan and cook for an additional 4-5 minutes.
Sprinkle with parsley and serve immediately
Notes
Lyonnaise potatoes are best if served hot or at room temperature. You can reheat leftover potatoes in the microwave for 1 minute or in a skillet for 5-10 minutes over high heat.