Add the chicken to a large bowl, then sprinkle with baking soda and cornstarch. Pour in the water and mix everything together until evenly coated. Set aside for at least 15 minutes.
In a large skillet or wok, heat the neutral oil over medium heat. Sear the sliced chicken in batches until fully cooked, then remove from the pan and set aside.
In the same wok, heat the sesame oil over medium heat. Sauté the onions and bell peppers until fragrant and slightly softened. Add the garlic and ginger and cook briefly until aromatic. Pour in the chicken broth, soy sauce, ketchup, dark brown sugar, and vinegar, stirring to combine. Let the mixture simmer until the flavors meld together, about 2 to 3 minutes.
Whisk together the cornstarch and water to make a slurry. Stir in the cornstarch and continue to simmer the sauce, stirring occasionally, until it thickens.
Add the sliced chicken back into the wok, stirring to coat it evenly in the sauce. Allow everything to simmer for 5 minutes to meld the flavors. Serve hot.