Season the meat with salt and pepper, then mix in the egg and cornflour until each piece is well coated. Roll each chunk in extra cornflour for a light, crispy finish.
Heat enough oil in a wok or deep frying pan to cover the pieces. Fry the meat in small batches for 3-4 minutes until golden and cooked through. Remove and drain on kitchen paper.
In a small saucepan, combine the ketchup, vinegar, soy sauce, sugar, pineapple juice and sesame oil. Bring to a simmer, then stir in the cornflour slurry to thicken slightly.
In a clean wok, stir-fry the bell peppers, onion and pineapple chunks (and ginger, if using) over high heat for 2-3 minutes, until just softened but still crunchy.
Add the fried meat back into the wok with the vegetables, pour over the sauce, and toss until everything is coated. Serve immediately with steamed jasmine rice or fried rice.