Heat a large skillet over medium heat and add the olive oil.
When the oil shimmers, add the diced onion. Sauté 3–4 minutes until softened and translucent.
Add the minced garlic and cook for 30–60 seconds until fragrant.
Add the ground turkey and break it up with a spatula. Cook for 6–7 minutes, stirring occasionally, until no pink remains and the turkey is nicely browned in places.
While the turkey cooks, whisk together soy sauce, 1/4 cup water, brown sugar, rice vinegar, sesame oil, and grated ginger in a small bowl.
In another small bowl, stir the cornstarch into 1/4 cup cold water until smooth to make a slurry.
Pour the soy-ginger sauce over the cooked turkey and stir. Bring to a gentle simmer and cook for 2–3 minutes to marry flavors.
Pour in the cornstarch slurry while stirring. Simmer for 1–2 minutes until the sauce thickens and becomes glossy.
If you like heat, stir in red pepper flakes to taste.
Spoon the teriyaki turkey over warm rice, sprinkle with sliced green onions and sesame seeds, and serve immediately.
Notes
Store leftovers in an airtight container for 3–4 days. For meal prep, keep turkey and rice separate.