How to Make It
Bring a medium pot of salted water to a boil. Stir in the rice and cook until tender, 10 to 12 minutes. Drain the rice well and transfer it to a large glass or stainless-steel bowl. Add 1/4 teaspoon of the salt, the sugar, vinegar, and lemon zest and fold in with a rubber spatula. Set aside to cool.
Meanwhile, using a vegetable peeler, shave lengthwise slices from the cucumbers; discard the seeds. Put the cucumber slices in a strainer set over a medium bowl and toss them with the remaining 1/2 teaspoon salt. Set aside for at least 10 minutes.
In a small frying pan, toast the sesame seeds over moderate heat, stirring frequently, until light brown, 2 to 3 minutes. Remove the seeds from the pan.
Add the cooking oil, horseradish, sesame oil, and half the smoked salmon to the cooled rice and fold gently to combine. Divide the rice mixture among four plates or put it on a large platter. Top with the remaining smoked salmon. Squeeze the cucumber slices to remove any excess liquid and then put them on the salads. Sprinkle with the toasted sesame seeds and the scallions.