In a large bowl, combine the chicken, soy sauce, ginger-garlic paste, salt, white pepper, cornstarch, flour, and water. Mix until the chicken is evenly coated. Let marinate for 20 to 30 minutes.
Heat 2 to 3 inches of vegetable oil in a deep skillet or Dutch oven to 350°F.
Fry the chicken in batches for 4 to 5 minutes, or until lightly golden and cooked through. Transfer to a wire rack and let rest for 5 minutes.
Fry the chicken a second time for 60 to 90 seconds until deeply golden brown and crispy. Return to the wire rack.
In a small bowl, whisk together the soy sauce, dark soy sauce, chili garlic sauce, ketchup if using, rice vinegar, sugar, white pepper, chicken broth, and MSG if using.
In another small bowl, stir together the cornstarch and water to make the slurry.
Heat a wok or large skillet over high heat. Add the vegetable oil.
Stir-fry the garlic, ginger, green chilies, celery if using, and scallion whites for about 30 seconds until fragrant.
Add the onion and bell peppers. Stir-fry for 2 minutes, keeping the vegetables slightly crisp.
Pour the sauce mixture into the wok and bring it to a gentle simmer.
Stir the slurry again and add it to the sauce. Cook for 20 to 30 seconds until the sauce becomes lightly glossy.
Add the fried chicken and toss quickly for 20 to 30 seconds until evenly coated.
Remove from the heat and stir in a handful of scallion greens.
Transfer to a serving platter and garnish with the remaining scallion greens. Drizzle with sesame oil if desired.
Serve immediately.