In a mixing bowl, whisk together egg white, 2 tablespoons of the cornstarch, 2 tablespoons of the dry sherry, 1 tablespoon soy sauce, and chili powder; whisk until well combined. Add the chicken and toss to coat. Cover and refrigerate for 2 hours.
In a mixing bowl, whisk 1 tablespoon of cornstarch with 1 tablespoon of the chicken stock; whisk until smooth. To the bowl, add the remaining chicken stock, 1 tablespoon dry sherry, 1 tablespoon soy sauce, rice vinegar, and sugar; whisk until combined and set aside.
In a large pot, heat vegetable oil to 350˚F.
Remove the chicken from the marinade and drop it into the hot oil. Fry the chicken, turning it once, until golden brown and cooked through, about 2 to 3 minutes. Remove and drain on paper towels. Set aside.
In a large skillet or wok, heat 1 tablespoon vegetable oil. Add the sliced red bell peppers and broccoli; cook until fork-tender, about 3 minutes. Add garlic, ginger, red pepper flakes and chopped green onions.
Continue to cook and stir occasionally for 1 minute; add the chicken stock sauce, bring to a boil, and cook, stirring, until the sauce thickens, about 1 minute. Remove from heat.
Arrange the cooked chicken on a platter and pour the sauce over it.
Serve over rice and garnish with green onions and sesame seeds.