First, peel the 3 pounds of potatoes and wash them well.
Cut the potatoes on the thicker side about 1/2 inch and put them in a glass bowl.
Peel the white onion and wash, and slice. Set aside.
Set the oven's temperature to 400 F.
In a pot of salted water, add the potatoes.
Bring the potatoes up to a boil and blanch for 2 minutes.
Remove the pan from the heat and drain.
In a large oven-proof saute pan, heat the olive oil.
Add the onions to the pan once the oil is heated and season with salt and white pepper.
For 10 to 12 minutes, sauté the onions on medium-low heat.
Add the garlic and heat until fragrant about 2 minutes more.
Turn the onion mixture into a bowl.
Place the pan back on the stove and melt the butter.
After the butter has melted, add 1/3 of the potatoes to the pan.
Cover the first layer of potatoes with 1/2 of the onions. Cover the onions with 1/3 of the potatoes.
Repeat the layering until all of the potatoes and onions are used.
Place the pan in the oven and cook for 10 to 12 minutes or until the potatoes are golden brown.
The pan needs to be removed from the oven. Using a spatula, gently lift the potatoes out of the pan.
Slice the lyonnaise into 12 slices. Garnish with parsley.