Using a box grater or a food processor with the shredding disk, grate potatoes and onion alternatively. Transfer mixture onto a thin, clean dish towel; gather the corners and holding it over a large bowl, squeeze out as much liquid as possible.
Don’t discard the liquid yet and let it sit for 2 minutes to let the starch settle to the bottom. A couple minutes later, discard the liquid but keep the starch deposit.
Put the potato mixture in the bowl. To this add beaten eggs, matzo meal, flour, salt, freshly ground black pepper and parsley; stir until well combined and set aside.
In a large cast iron pan over medium-high, heat oil and duck fat until it reaches 350ºF. Meanwhile, using a ¼ cup measuring cup, portion the potato mixture and flatten them down to about ½-inch thick.
When oil reaches the right temperature, gently place the latkes making sure not to overcrowd the pan. Fry them until golden brown, about 3 minutes per side flipping them carefully. When cooked, transfer them to a wire rack sitting on a baking sheet lined with paper towel. If making a large batch, keep latkes in the oven at 170ºF. Serve with apple sauce and crème fraiche.