Preheat the oven to 200°. Using a box grater or a food processor fitted with the shredding disc, grate the potatoes. Working in batches, transfer the potatoes to a kitchen towel, wrap tightly, and squeeze out the excess liquid (if the potatoes turn brown, it’s okay). In a large bowl using your hands, mix the potatoes, onion, if using, the eggs, and 2 large pinches fine sea salt.
Add enough oil to a large skillet to measure 1/4 to 1/2 inch; heat over medium-high. When the oil begins to shimmer, working in batches, scoop out the potato mixture by the 1/3 cupful, form into patties, draining any excess liquid back into the bowl, and carefully add to the skillet. Using the back of a spatula, press the latkes to flatten. Cook until golden and the edges are dark brown and lacy, 2 to 3 minutes per side. Adjust the heat as needed to prevent the potatoes from burning. Transfer to a rack set inside a rimmed baking sheet and top with flaky sea salt; transfer to the oven to keep warm. If the oil looks too dark brown between batches, pour it into a heat-proof container, wipe the skillet clean, and reheat a new batch of oil before continuing.
Arrange the latkes on a platter. Serve with the chimichurri, salmon, apple sauce and sour cream. Sprinkle with the dill.