Fill a large bowl with cold water. Working one at a time, peel potatoes and grate on the large holes of a box grater. Place grated potatoes into the bowl with cold water.
Once the potatoes have been added to the water and soaked for 1-2 minutes, drain and place in a large clean kitchen towel with the grated onion and squeeze to remove as much liquid as possible. Place potatoes and onion in a large bowl and combine with Everything Bagel Seasoning, salt, pepper, egg and flour.
Heat 1/4 cup oil in a large nonstick skillet. Spoon potatoes into skillet, making them into about 3-inch wide and 1/2-inch tall cakes. Cook until crisp and golden, about 4-5 minutes per side. Transfer to a towel-lined place. Repeat with remaining ingredients, adding more as needed to crisp the latkes.
To assemble the “Double Down” cakes, spread sour cream on one side of a latke and top with smoked salmon, onions and a slice of tomato. Spread more sour cream on another latke and top the sandwich. Repeat with remaining ingredients.
To Make the Everything Bagel Seasoning:
In a small bowl, mix ingredients to combine well. Store in an airtight container.