In a large mixing bowl, combine the cubed chicken with ginger-garlic paste, 1 tsp soy sauce, 1/2 tsp black pepper, and 1/4 tsp salt. Mix well to ensure all chicken pieces are coated. Cover and marinate in the refrigerator for at least 20 minutes, or up to 2 hours for best flavor.

Preheat your air fryer to 400°F (200°C).

In a separate shallow bowl, whisk together all-purpose flour and cornstarch. In another shallow bowl, lightly beat the egg.

Dip each marinated chicken piece first into the beaten egg, allowing excess to drip off, then dredge thoroughly in the flour-cornstarch mixture, ensuring it's fully coated. Gently shake off any excess coating.

Lightly spray the bottom of your air fryer basket with neutral oil. Arrange the coated chicken pieces in a single layer in the air fryer basket, ensuring not to overcrowd. You will likely need to cook in batches. Lightly spray the tops of the chicken pieces with oil.

Air fry for 8-10 minutes, then flip the chicken pieces, spray again lightly with oil, and continue to air fry for another 7-9 minutes, or until the chicken is golden brown, crispy, and cooked through (internal temperature 165°F/74°C). Repeat with remaining chicken. Set aside the crispy chicken.

While the chicken is air frying, prepare the chili sauce. In a small bowl, whisk together the 3 tbsp soy sauce, vinegar, sweet chili sauce (if using), ketchup, sugar, and 1/2 tsp black pepper. Set aside.

Heat 1 tbsp neutral oil in a large wok or skillet over medium-high heat. Add minced ginger, minced garlic, and slit green chilies. Sauté for 30 seconds until fragrant.

Add the diced onion and bell pepper. Sauté for 2-3 minutes until the vegetables are slightly softened but still retain some crunch.

Pour in the prepared chili sauce mixture and stir well. Add 1/2 cup water or chicken broth and bring to a simmer. Let it cook for 1-2 minutes.

Stir in the cornstarch slurry (1 tbsp cornstarch mixed with 2 tbsp water). Continue to cook, stirring constantly, until the sauce thickens to your desired consistency. If it becomes too thick, add a splash more water or broth.

Add the air-fried crispy chicken to the wok with the sauce. Toss gently to coat all the chicken pieces evenly. Cook for another 1-2 minutes, just to heat through and allow the flavors to meld.

Remove from heat. Garnish with chopped spring onion whites and greens, fresh cilantro, and toasted sesame seeds, if desired.

Serve immediately with steamed white rice, noodles, or fried rice.