How to Make It
Preheat the oven to 200°F.
In a food processor fitted with the shredding blade, grate the potatoes. (You can also grate them by hand, but it is very time-consuming.) Transfer the potatoes to a large bowl. Grate the onions and add them to the bowl with the potatoes. Add the salt. Using your hands, thoroughly mix the potatoes, onions, and salt. Set aside.
In a deep-sided skillet, heat ½ inch of olive oil over medium-high heat until the oil begins to shimmer and show small bubbles.
While the oil is heating, form the potato pancakes. Using a ⅓-cup measuring cup, scoop out enough shredded potato mix to fill the cup. Hold the cup over a separate bowl and press as much liquid as possible out of the potatoes. Tap the contents of the cup into the palm of your hand and shape it into a thick pancake, pressing as much additional liquid from the pancake as possible. Set the latke on a large plate. Repeat until you have used up all the potato mix. (You may need 2 large plates to hold all the pancakes. Discard the pressed liquid from the potatoes.)
When the oil is hot enough, cook the latkes in batches. Use a spatula to lift the latkes off the plate and gently set them in the boiling olive oil. (If the oil starts to smoke or splatter, reduce the heat slightly.) Do not crowd the pancakes. Fry until the latkes are crisp on the outside and both sides are dark tan or very golden brown, about 10 minutes per side.
Transfer each batch of cooked latkes to a paper towel-lined baking dish and keep warm in the preheated oven until ready to serve.
To serve, top each latke with a dollop of sour cream. Place a small spoonful of masago on top of the sour cream. Sprinkle with the green onions. Garnish with pepper to taste.