Step 1

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).

Step 2

Meanwhile, slice potatoes crosswise 1/4-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.

Step 3

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.