Wash and dry the zucchini. Trim the ends and cut into evenly sized 2-inch sticks.
Heat the oil and butter in a large skillet or wok over medium-high heat. Add the onion and cook for 2 minutes.
Add the garlic and ginger and stir for 30 seconds until fragrant.
Add the zucchini in a single layer and cook for 2 minutes without stirring too often.
Stir and cook for 3 to 4 minutes, until the zucchini is tender-crisp and lightly browned.
Add the soy sauce, teriyaki sauce, black pepper, and red pepper flakes. Toss well and cook for 1 minute.
Remove from heat and garnish with sesame seeds and green onions. Serve immediately.