Heat a large skillet over high heat and add 1 tablespoon avocado oil.
Add the cubed steak and chopped red onion to the hot skillet and cook, stirring occasionally, until the beef begins to brown, about 4–6 minutes.
Add the chopped zucchini and continue to cook, stirring occasionally, until the zucchini softens and the beef is browned, about 5–7 minutes more.
Pour in 1/4 cup teriyaki sauce (or more to taste) and reduce heat to medium-high; cook, stirring occasionally, until most of the liquid has evaporated and a thick sauce coats the beef and vegetables, about 8–12 minutes.
Taste and season with sea salt and additional teriyaki sauce if desired.
Serve immediately over rice or with your choice of sides.