Peel potatoes and coarsely grate, directly into a large bowl of cold water.
Soak potatoes 1 to 2 minutes after last batch is added to water, then drain well in a colander.
Spread grated potatoes, garlic and onion on a kitchen towel, roll up and wring out as much liquid as possible.
Transfer potato mixture to a bowl and stir in egg and salt. If it’s too runny add a little flour or matzo meal. Too dry add a little bit more egg.
Heat a large skillet. Add enough oil to cover the bottom plus a bit more. Get the oil hot but not smoking. Use a spoon to potato mixture into skillet, spreading with the back of the spoon into 3 or 4-inch rounds.
Reduce heat to moderate and cook until undersides are very well browned, about 5 minutes. Turn latkes over and cook until undersides are very well browned, about 5 minutes more. You want walnut brown, not beige.
Transfer to paper towels to drain and season with salt. Add more oil to skillet as needed.
Keep latkes warm on a wire rack set in a shallow baking pan in oven.
To make the Double Down, spread crème fraiche on one side of a latke, pile on lox and onions, spread some crème fraiche on another latke, then make into a sandwich.