Make the dressing: Place the sesame oil, rice vinegar, sesame seeds, sugar, soy sauce and red-pepper flakes into a small bowl. Add 1 tablespoon of water and whisk until emulsified. Taste, and adjust seasonings. (Dressing should taste acidic, a little sweet and salty.)
When you’re ready to eat, prepare the salad: Halve the avocados and discard the pits. Using a small paring knife, carefully score the avocado flesh into ½-inch cubes, avoiding cutting through the skin. Use a large spoon to scoop out the avocado flesh (in one spoonful, if possible), as close to the skin as possible. Transfer avocado cubes to a large bowl and add the dressing; toss gently.
Add the cucumbers and scallions and toss everything together. Season with salt and black pepper, top with more sesame seeds, and serve immediately.
