Meanwhile, put 1 tablespoon of clarified butter in a saute pan over medium heat. Add the sliced onion and saute until golden brown, about 10 minutes. Stir in the crumbled bacon. Set aside.
With the julienne blade of a mandoline or the large holes of a shredder/grater, shred the potatoes. By handfuls, squeeze any excess liquid from the shreds and transfer them to a mixing bowl. Add the remaining butter and the salt and pepper and mix well.
Heat a 10-inch nonstick ovenproof skillet over medium heat. Add the olive oil. Squeezing away any liquid that has accumulated in the bowl, arrange half of the potatoes in an even layer. Spread the sauteed onion over the potatoes, and then top with the remaining potatoes in an even layer. Saute over medium heat until the underside of the potatoes are golden brown, 10 to 15 minutes.
To flip the cake, slide it out of the skillet browned side down onto a large plate, then invert the skillet over the plate and flip both together to return the cake to the skillet browned side up.
Put the skillet in the oven and bake until the cake is cooked through, about 10 minutes more. The potatoes should feel tender when the cake is pierced with a skewer or a sharp knife tip. With a spatula, lift 1 side of the cake slightly to check if the underside is well browned. If not, put the skillet on top of the stove over medium-high heat and cook briefly to brown the underside.
Slide the potato cake onto a serving platter. Garnish with parsley and, with a sharp knife, cut it into 6 wedges. Serve immediately.