Cut the chicken breasts into 1cm thick slices. Mix the egg yolk, water, flour, rice wine and soy sauce. Pour this mixture over the chicken strips and marinate for 15 minutes.
Put some cornstarch in a shallow bowl. Roll each chicken strip in it until evenly coated. Place them onto a plate next to each other.
Heat an inch of oil in a hot wok. When it's hot (and starts to smoke), fry the chicken bits in batches until they change color, then drain them on paper kitchen towels. Let them rest whilst you prepare the lemon sauce.
In a saucepan mix the lemon juice, sugar and rice wine, then cook and stir until the sugar melts. Mix the extra water with the cornstarch and add to the syrup. Bring to the boil and stir for 1-2 minutes until you get a thick, smooth syrup. Turn off the heat and keep warm.
Pour away the oil from the wok and wipe it clean. Heat it up again, and, when hot, pour in fresh oil and heat to smoking point. Stir-fry the chicken strips again (in batches) for about 2 minutes until crispy and brown. Drain on paper towels.
Serve in a large bowl - pour over the lemon syrup and garnish with the spring onion slices.