Prepare the Chicken: Season the chicken pieces with a little salt and pepper.
Sauté Aromatics: Heat sesame oil in a large pot over medium heat. Add the onion, garlic, and ginger. Sauté for 2-3 minutes until fragrant.
Cook the Chicken: Add the chicken to the pot. Cook for 5-7 minutes, stirring occasionally, until browned on all sides.
Add Vegetables: Stir in the carrots, bell pepper, and mushrooms. Cook for another 3-4 minutes until the vegetables start to soften.
Simmer the Broth: Pour in the chicken broth and soy sauce. Bring to a simmer, then cover and cook for 10-15 minutes until the chicken is fully cooked and the vegetables are tender.
Thicken the Broth: In a small bowl, mix the cornstarch with a little water to form a slurry. Gradually pour it into the pot while stirring. Let the mixture cook for 2-3 minutes until thickened.
Season and Serve: Taste and adjust seasoning with salt, pepper, or chili flakes if desired. Garnish with green onions before serving.