Make the sauce: In a small bowl, whisk soy sauce, brown sugar, rice vinegar, mirin, water, cornstarch, and red pepper flakes.
Set aside. The cornstarch will settle—give it a quick stir before using.
Prep the veggies: Dice the onion, mince the garlic and ginger, and slice the bell pepper, broccoli, and carrots. Keep them ready by the stove so the cooking moves fast.
Brown the turkey: Heat oil in a large skillet over medium-high heat.
Add ground turkey, breaking it up with a spoon. Season lightly with salt and pepper. Cook until no longer pink and lightly browned, about 5–6 minutes.
Add aromatics: Stir in onion, garlic, ginger, and the white parts of the green onions.
Cook 2–3 minutes until fragrant and the onion softens slightly.
Cook the veggies: Add bell pepper, broccoli, and carrots. Stir-fry 3–4 minutes until crisp-tender. Don’t overcook; they’ll soften more once the sauce goes in.
Sauce it up: Give the teriyaki sauce a quick whisk and pour it into the skillet.
Stir constantly as it bubbles and thickens, 1–2 minutes. If it gets too thick, splash in a bit of water to loosen.
Finish with sesame oil: Turn off the heat and stir in sesame oil for a toasty finish. Taste and adjust with a pinch of salt, extra vinegar for brightness, or a touch more sweetener if needed.
Garnish and serve: Sprinkle with the green parts of the green onions and sesame seeds.
Serve hot over rice or noodles. A squeeze of lime perks everything up.