Preheat grill to medium-high and brush grill grates with oil.
In a saucepan set over medium heat, combine soy sauce, sesame oil, garlic, and sugar; whisk and bring to a simmer, stirring frequently, until it begins to thicken. Remove from heat. Set aside.
Cut the zucchini into long 1/4-inch thick strips.
Place zucchini on the hot grill in one single layer.
Close the lid and cook for 3 minutes.
Using tongs, flip over the zucchini; cover and continue to cook for 2 to 3 more minutes, or until charred and tender. Remove from heat.
Remove from heat and brush each slice of zucchini with the sauce.
Garnish with sesame seeds and pepper flakes. Serve.
Notes
To cut long slices of zucchini, using a mandoline slicer is the best approach. It allows you to set your desired thickness and makes slicing quick and easy. Always use the hand guard or a cut-resistant glove for safety because the blades are extremely sharp.
Adjust Spices: Add more red pepper for heat or reduce sugar to suit your taste.
Preheat and Oil the Grill: For char marks and to prevent sticking.
Marinate Zucchini: If possible, let it sit for 15 minutes after applying the marinade for extra flavor.