Heat the vegetable oil in a large pot over medium heat.
Season chicken thighs generously with black pepper.
Add the chicken thighs to the pot and brown on both sides, about 5-7 minutes total.
Remove the chicken and set aside.
Add sliced onions to the pot and cook until translucent, about 3-4 minutes.
Stir in minced garlic and cook for another minute.
Return the chicken to the pot; pour in chicken broth, soy sauce, and add sugar.
Bring the mixture to a simmer and cover, cooking for 20 minutes.
Uncover, add cornstarch, and stir until the sauce thickens.
Serve hot with a sprinkle of green onions on top.