Shred the onion and potatoes in a food processor fitted with a medium-shredding blade or on the large holes of a box grater.
Tip the potato mixture onto a clean kitchen towel. Squeeze out as much liquid as you can and discard the liquid.
Tip potato mixture into a large bowl and mix in flour, everything bagel seasoning, salt and pepper.
Heat a medium non-stick saucepan over medium heat. Add 4 tbsp olive oil followed by the latke batter. Press into an even layer and cook for 5-7 minutes, or until the edges look deeply golden.
In a small bowl, mix together the scallion sour cream ingredients and set aside.
Carefully tip out excess oil from the latke into a small bowl, using the spatula to support the latke so it doesn’t slip out. Flip latke out onto a cutting board.
Slide latke back into the pan. Add 2 tbsp olive oil around the edge of the pan. Cook for another 5-7 minutes. Flip latke onto a serving tray.
Finish by spreading on the scallion sour cream and topping with the smoked salmon, red onion, dill, capers, and a sprinkle of everything bagel seasoning.