Bring a large saucepan of lightly salted water to the boil. Add the noodles and boil for about 6 minutes, or according to the packet instructions, until soft. Drain and set aside.
Meanwhile, make up the lemon juice to 1/2 cup (125ml) with water, if necessary, then blend with the cornflour in a bowl or jug until smooth. Stir in the stock, soy sauce and sugar, then add the lemon zest and chilli flakes. Set aside.
Heat a wok or large frying pan over a high heat. Add the cashew nuts and stir for about 1 minute until browned in places. Tip them out and set aside.
Return the wok to the heat and add the oil, swirling it around to coat the sides. Add the onions and garlic and stir-fry for 3 minutes or until tender. Add the chicken pieces and stir-fry for 2–3 minutes until the chicken is cooked through and beginning to brown.
Pour in the reserved lemon-flavoured sauce and add the broccoli florets and ginger. Bring to the boil, then cover and simmer for 4–5 minutes until the broccoli florets are tender but still crisp.
Add the noodles to the wok. Use two large forks to mix them in with the other ingredients. Scatter the coriander and cashew nuts over the top, then serve.