Combine the dressing ingredients in a small bowl, you may need to add a little more water to get the right consistency.
Partially peel the cucumber, cut in half lengthwise, remove the seeds, then slice about 1/4″ thick depending on your preference. Remove the stem and root of the radish and slice thinly, or use a peeler. Roughly chop the green onions and finely slice the red onion.
Mix all the cucumber, radish, red onion, and green onion. Toss with dressing. Garnish with avocado and black sesame seeds, serve immediately.
If you are packing this for your lunch, make sure to pack the dressing separately because if it sits too long in the dressing the veggies get soggy and the dressing turns watery.