Heat sesame oil in a large skillet over medium-high heat. Add the garlic and ginger, sautéing for 30 seconds until fragrant.
Add the ground chicken and cook, breaking it up with a spoon, until browned and fully cooked through—about 5–7 minutes.
In a small bowl, whisk together soy sauce, hoisin sauce, rice vinegar, brown sugar, water, and red pepper flakes.
Pour the sauce into the skillet with the chicken. Stir and bring to a simmer.
Add the cornstarch slurry to the pan and continue cooking until the sauce thickens and coats the chicken, about 2–3 minutes.
Serve hot over steamed rice and garnish with sliced green onions and sesame seeds if desired.