For the caramelized onion sour cream: Heat the oil in a large skillet over medium heat and add the onion, sugar and 1/4 teaspoon of the salt. Cook, stirring, until the onions are golden brown and caramelized, 30 to 40 minutes. Add the garlic and cook for 2 more minutes. Let cool.
Mix together the onion mixture, sour cream, onion powder, a few turns of black pepper and remaining 1/4 teaspoon salt. Taste and adjust the seasoning as desired. Cover and refrigerate until ready to serve.
For the potato latkes: Meanwhile, using the shredding attachment on a food processor (or mandoline), shred the potatoes, onions and garlic. Line a colander with 2 layers of cheesecloth and set it over a bowl or in the sink. Add the shredded mixture to the cheesecloth and sprinkle with the kosher salt. Let sit for 30 to 45 minutes.
Gather up the edges of the cheesecloth and use your hands to squeeze out any excess moisture.
In a large bowl, mix together the breadcrumbs, lemon juice, eggs, a few turns of black pepper and the potato mixture. Stir in the scallions.
In a large skillet, heat 1/4 inch of oil over medium-high heat until shimmering; it's ready when a strand of potato added to the oil immediately starts to sizzle. Use a 1/2-cup ice cream scoop to add compact scoops of the mixture to the oil, spacing them out so as not to crowd the pan. Press the scoops down lightly with a spatula to get 1/2-inch-thick patties. Fry until golden brown on both sides, a few minutes per side. Transfer to a wire rack or paper towels and sprinkle with flaky or kosher salt. Repeat with the remaining mixture, adding more oil to the pan as needed between batches.
Sprinkle the caramelized onion sour cream with chopped scallions and serve with the warm latkes.