Prep. Preheat the grill to medium-high and brush the grill grates with oil. Cut the zucchini into long, 1/4-inch-thick strips and set aside.
Make the glaze. In a saucepan set over medium heat, combine the soy sauce, sesame oil, garlic, and sugar; whisk and bring to a simmer, stirring frequently, until it begins to thicken. Remove from heat and set aside.
Grill. Place the zucchini on the hot grill in a single layer. Close the lid and cook for 3 minutes. Using tongs, flip the zucchini over; cover and continue to cook for 2 to 3 more minutes, or until charred and tender. Remove from heat.
Finish and serve. Brush each slice of zucchini with the prepared glaze sauce. Garnish with sesame seeds and pepper flakes and 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.