In a small bowl, whisk together soy sauce, rice vinegar, brown sugar, ginger, and garlic. In another bowl, mix cornstarch with water until dissolved. Combine both mixtures and set aside.
Heat half a tablespoon of avocado oil in a wok or large skillet over high heat. Add onion and red bell pepper, seasoning with salt and pepper. Stir and cook for 3-5 minutes until softened.
Add broccoli florets and water to the pan. Cover tightly and steam for 2-3 minutes until crisp-tender. Remove vegetables and keep warm.
Add remaining avocado oil to the pan. When hot, add flank steak, seasoning lightly with salt and pepper. Stir constantly for 1-2 minutes until cooked through.
Return vegetables to the pan and pour teriyaki sauce over. Bring to a boil, then reduce heat and simmer for 2-3 minutes until sauce thickens.
Serve hot over steamed white rice, garnished with sesame seeds.
Notes
Store leftovers in an airtight container in the refrigerator for 3-4 days. Reheat gently on the stovetop or microwave.