Cut the chicken into bite-sized pieces and place them in a large bowl. Toss the chicken with ½ cup of cornflour (first measurement), then add the egg and mix everything together until well combined. Set this aside while you prepare the lemon sauce. In a separate bowl, whisk together the lemon juice, brown sugar, honey, crushed garlic, soy sauce, and chicken stock. Heat the vegetable oil in a large frying pan over medium heat. Fry the chicken pieces in batches until they are crispy and golden on both sides. Remove the cooked chicken from the pan and set aside. Wipe out any excess oil from the frying pan, then pour in the lemon sauce mixture and let it simmer for about 2 minutes. In a small bowl, dissolve 1 tablespoon of cornflour (second measurement) in 50ml of water, then stir this slurry into the simmering sauce. Once the sauce begins to thicken, return the crispy chicken to the pan and toss to coat each piece thoroughly in the sticky lemon sauce. Garnish with sliced spring onion and sesame seeds. Serve immediately alongside hot rice and steamed green vegetables.