In a large non-stick skillet, heat 1 tablespoon of avocado oil and 1 tablespoon of butter over medium heat until sizzling. Add sliced onions with a pinch of salt, stirring occasionally. Cook for 15-20 minutes until golden brown and soft, then set aside.

Slice the Yukon Gold potatoes crosswise into 1/4-inch thick rounds. Boil salted water, add potato slices, and parboil for about 3 minutes until nearly tender. Drain thoroughly.

Return the skillet to heat, adding the remaining avocado oil and butter. Once bubbly, add parboiled potato slices in a single layer. Cook for 15-18 minutes, turning every 3 minutes until golden and crispy.

Return caramelized onions to skillet, seasoning with salt and pepper to taste. Stir for about 2 minutes until heated through and well combined. Transfer to serving platter, garnish with parsley, and serve warm.