Wash and trim the zucchini, cut into 2-inch pieces, then slice into wedges while cutting the onion into large chunks.

Heat oil in a wok or large skillet over medium-high heat until shimmering.

Add zucchini and onion, spreading them out to sear and cook for about 2 minutes with occasional stirring.

Pour in teriyaki sauce and soy sauce, tossing quickly to coat the vegetables evenly.

Add red pepper flakes and black pepper, stirring as the sauce begins to form a light glaze.

Cook for another 3–4 minutes until zucchini is tender but still slightly firm.

Remove from heat, sprinkle with sesame seeds, and serve immediately.