Whisk the soy sauce, water, honey, rice vinegar, garlic, ginger, sesame oil, cornstarch, and red pepper flakes in a small bowl until smooth. Set aside.
Heat the avocado oil in a large skillet or wok over medium-high heat. Add the ground turkey and onion. Cook for 6 to 8 minutes, breaking up the meat, until no longer pink and the turkey reaches 165°F (74°C).
Add the broccoli, bell pepper, and carrots. Stir-fry for 4 to 5 minutes, until the vegetables are bright and crisp-tender.
Whisk the teriyaki sauce again, pour it into the skillet, and stir constantly for 2 to 3 minutes until bubbling and thick enough to coat the turkey and vegetables.
Remove from the heat. Top with green onions and sesame seeds, then serve over brown rice, cauliflower rice, or noodles.