Combine soy sauce, brown sugar, minced garlic, grated ginger, rice vinegar, and sesame oil in a mixing bowl. Whisk until the sugar dissolves and the mixture is smooth. Set aside.
Trim excess fat from the chicken thighs if needed. Cut each piece into two or three large chunks for even cooking.
Place the chicken pieces into the slow cooker, spreading them out evenly. Pour the prepared sauce over the chicken, making sure all pieces are coated well.
Cover the slow cooker with the lid and set it to low. Let it cook for 4 to 6 hours until the chicken is very tender and falling apart, filling your home with a savory aroma.
Once cooked, use a slotted spoon to carefully remove the chicken thighs from the slow cooker. Place them on a plate and tent loosely with foil to rest for 5 minutes.
Pour the cooking liquid into a small saucepan. Bring it to a gentle simmer over medium heat. Dissolve the cornstarch in a teaspoon of cold water, then whisk into the simmering sauce. Cook for 2-3 minutes until the sauce thickens to a glossy, clingy consistency.
Slice the rested chicken into strips or serve whole, then drizzle generously with the thickened sauce. Garnish with sliced green onions and a sprinkle of sesame seeds for crunch and flavor.
Serve hot over steamed rice or alongside your favorite vegetables for a cozy, satisfying meal.
