In a large bowl, place chicken and marinade ingredients. Mix thoroughly to break up the egg white. Allow to marinate for 20 minutes. ​ In another bowl, mix together the chicken stock, soy sauce and sugar. ​ In a separate bowl, mix together the lemon juice and zest. ​ Place the flour in a large tray or bowl. Add the chicken pieces and the marinade and use your hands to pat and press the liquid and flour together around the chicken pieces to form a craggy coating. ​ Fill a wok or saucepan to about 1/3 capacity with the vegetable oil. Heat over high heat. The oil is hot enough for frying at 325°F or 165°C or when a wooden spoon dipped into the oil forms small little bubbles. Shake excess flour from the chicken pieces. Cook chicken pieces in 2 to 3 batches for about 3 minutes or until golden and cooked through. Drain on kitchen paper. ​ In a clean wok or pan over medium-high heat, add 1 tablespoon of oil and then add the garlic and ginger. Cook for about 10 seconds or until fragrant. Then add the chicken stock mixture. Cook for 2-3 minutes until the sugar has dissolved. Then add the lemon juice and zest. Simmer for another 30 seconds before adding the cornstarch mixture. Cook for another 30 seconds or until sauce has thickened slightly. Add the chicken pieces and toss until evenly coated. Transfer to a serving plate and sprinkle with sesame seeds and spring onion. Garnish with lemon slices if using.