Combine the chicken broth, soy sauce, brown sugar, white vinegar, garlic, ginger, and red pepper flakes in a medium bowl and whisk until well combined.
In a small bowl, whisk together the cornstarch and cold water until well combined.
Heat the vegetable oil in a large skillet over medium-high heat. Add the bell pepper and onion and cook until softened, about 5 minutes.
Add the chicken thighs to the skillet and cook until lightly browned, about 3 minutes per side.
Transfer the chicken thighs to a slow cooker. Pour the sauce over the chicken and add the cooked vegetables. Stir to combine.
Cover the slow cooker and cook on low heat for 4-5 hours or until the chicken is cooked through and tender.
Once the chicken is cooked, add the cornstarch slurry to the slow cooker and stir to combine. Cover and cook for an additional 15 minutes or until the sauce has thickened.
Serve the chicken with rice or your favorite side dish.
Interesting Facts
Sweet and sour chicken is a classic Chinese dish that is popular around the world.
The dish typically consists of chicken, bell peppers, onions, and a sweet and sour sauce.
The addition of red pepper flakes gives the dish a hint of heat.