Combine the marinade ingredients and toss with the sliced steak; let marinate for at least 15 minutes.
Whisk together the sauce ingredients in a separate bowl and set aside.
Heat avocado oil in a wok or large skillet over high heat and sear the marinated steak until browned; remove and set aside.
In the same pan, stir-fry the bell peppers and onion until crisp-tender.
Return the steak to the pan, pour in the sauce, and toss until it thickens and coats everything.
Serve hot over steamed rice.