Toss the chicken pieces with vegetable oil and salt in a large bowl and let them sit for 10 minutes to soak up the flavors.
Sprinkle cornstarch over the chicken and toss until the pieces are unevenly coated, leaving some areas dusted.
Arrange the chicken in a single layer inside the air fryer basket without crowding and mist with oil to cover all surfaces.
Cook the chicken at 450°F (230°C) for 8 minutes until it looks pale yellow, then turn or shake the basket and air fry for another 5 to 6 minutes until golden brown.
Transfer the cooked chicken to a plate and let it rest briefly while you prepare the sauce.
Whisk all the sauce ingredients together in a small bowl until smooth.
Heat the peanut or vegetable oil in a large skillet over medium heat until shimmering.
Add minced garlic and ginger, stirring frequently until they smell fragrant.
Toss in the chopped bell pepper and onion, cooking them for about 30 seconds to soften slightly.
Stir the sauce mixture to reblend the cornstarch and pour it into the skillet, stirring constantly until the sauce thickens.
Return the chicken pieces to the skillet and mix well to coat everything evenly with the sauce.
Serve the sweet and sour chicken immediately while hot as a satisfying main dish.