Start by peeling the potatoes and washing them thoroughly. Cut them into thick slices, about ½ inch, and place them in a bowl.

Peel and wash the onions, then slice them evenly and set aside. Preheat your oven to 400°F.

In a large pot of salted water, add the potatoes and bring to a boil. Blanch the potatoes for 2 minutes only. Drain well and set aside.

Heat olive oil in a large oven-proof sauté pan over medium heat. Add the onions, season with salt and white pepper, and cook on medium-low heat for 10 to 12 minutes until soft and lightly golden. Add the garlic and cook for another 2 minutes until fragrant. Remove the onion mixture and set aside.

In the same pan, melt the butter. Add one-third of the potatoes in an even layer, followed by half of the onions. Add another layer of potatoes, then the remaining onions, and finish with the last layer of potatoes.

Transfer the pan to the oven and bake for 10 to 12 minutes until the potatoes turn golden. Carefully remove from the oven, gently lift with a spatula, garnish with parsley, and serve warm.