Clean chicken breasts and cut into 1-2 cm pieces. Whisk egg whites, garlic and soy sauce and marinade the chicken pieces for at least 1 hour. In the meantime, prepare the ingredients for the sauce. Peel the onion and cut into 1 cm pieces, then cut the peppers and spring onions into thin strips. Heat plenty of oil in your fryer or a deep pan on the stove. Roll the chicken pieces in potato flour so they're evenly covered. Shake off any excess. To prepare the sauce, heat sesame and grape seed oil in a wok or other suitable pan, fry the onions for 2 minutes until just softening. Add ginger, spring onions and pepper medley and continue to fry for 2 minutes. Stir in the tomato ketchup, vinegar, soy sauce, sugar and corn starch and pour over the chicken stock. Season with salt and leave to thicken. In the meantime, fry the chicken pieces in the hot oil for about 10 minutes until crispy and browning. Just before serving, fold into the sweet & sour sauce and offer with fluffy steamed basmati rice.