Slice cucumbers very thinly. A mandolin is best for this.
Place sliced cucumbers in a colander and sprinkle with the kosher salt. Toss so they are well coated. Set the colander in the sink so they drain. Let stand 15-20 minutes.
Lay the cucumber slices on a kitchen towel and pat to dry; place them in a large bowl then add the green onions and cilantro.
For the dressing: Mix the sugar, ginger, garlic and soy sauce in a small bowl until sugar is dissolved. Whisk in the oils then pour over cucumber slices and toss to coat.
Refrigerate until ready to use. This salad will keep about 2 days but the dressing will get a bit watery (but still taste good).
Notes
Don’t use regular cucumbers (store bought) because they are waxed. Kirby’s (pickling cucumbers) are a good substitute for the European ones.