2tablespoonsavocado oilor other high heat oil, divided
2tablespoonsbutterideally high fat, European style butter, divided
2mediumyellow onionsor sweet onions, halved top to bottom and thinly sliced
kosher saltto taste
1 ½poundsYukon Gold potatoesor peeled russet potatoes, (see recipe note)
freshly ground black pepper
¼cupfinely chopped fresh parsley
Heat one tablespoon each of avocado oil and butter in a large, non-stick skillet over medium heat until sizzling. Add onions and a generous pinch of salt; cook, stirring often until soft and lightly golden, 15-20 minutes. Remove from heat and transfer to a bowl (set skillet aside for potatoes - no need to wipe it out).
Meanwhile, slice potatoes crosswise ¼-inch thick and place in medium saucepan (if you have a mandoline slicer, this is a great time to use it). Add enough cold water to cover along with a generous pinch of salt; bring to boil over medium-high heat. Boil until nearly tender, about 3 minutes, skimming foam as needed. Drain and set aside.
Place the same large, non-stick skillet over medium heat and add remaining avocado oil and butter and heat until sizzling. Add parboiled potato slices and cook 15-18 minutes, gently flipping every 3 minutes to crisp potato slices on both sides. Once potatoes are crisp to your liking and cooked through, return onions to skillet and season to taste with salt and pepper; cook and stir until heated through, about 2 minutes. Transfer to platter and sprinkle with fresh parsley; serve hot.
Starchy baking potatoes are the traditional potato used in this dish, but I prefer Yukon Gold potatoes for their blend of waxy / starchy texture. They don't need to be peeled, crisp up nicely, and hold their shape better than russets. But either work well.