Cut in half, and lightly smash the cut side with the blade of a cleaver or chef's knife, until it starts to split.
Cut the cucumber into bite-sized chunks.
Place the cucumber into a bowl and sprinkle with coarse sea salt. Massage the cucumber with the sea salt and let it sit for 15 minutes to draw out excess moisture.
After 15 minutes, drain the excess liquid from the cucumber and pat dry with paper towel.
Make the dressing:
In a bowl, combine soy sauce, rice vinegar, sugar, sesame oil, and chili oil, if using.
Mix well.
Assemble:
Add the dressing to the cucumber and toss until well coated.
Let the cucumbers marinate in the fridge for 10-15 minutes.
Sprinkle with sesame seeds and serve as a side dish.