Wash the cucumbers. Cut each one in half lengthwise, then lay flat and smash firmly with the side of your knife or a rolling pin — you want them to crack open and split. Cut in half again lengthwise, then cut on the diagonal into rough 1-inch pieces.
In a small bowl, combine all the dressing ingredients — garlic, sugar, salt, MSG, soy sauce, rice vinegar, and sesame oil — and stir until the sugar and salt dissolve.
Drizzle the dressing on cucumber, add sesame seeds, and cilantro if using. Toss very gently with the cucumbers and serve immediately. Add chili oil if you prefer.
This is a dish that wants to be eaten the moment it is dressed. You may still enjoy it after it sits for a while, but the cucumber will release some liquid and the flavor will become slightly lighter, though still very tasty.