To make the marinade: cut the chicken fillets into bite sized chunks. Combine the brown sugar, cornflour, ginger, salt, sherry, soy sauce and lemon juice in a bowl and mix until smooth. Add the chicken chunks and toss until they are evenly coated in the marinade. Set aside.
To make the sauce: Combine the cornflour, soy sauce and chicken stock in a bowl. Set aside.
Heat one teaspoon of olive oil in a frying pan. Add the chicken and marinade and fry until the chicken is just cooked. Transfer the chicken to a plate and set aside.
Cook the noodles as per the instructions on the pack. While the noodles are cooking you can start cooking the veggies.
Heat another teaspoon of olive oil in a large frying pan. Add the garlic paste and cook for 30 seconds. Turn up the heat and fry the shiitake and portabellini mushrooms for 5 minutes. Add the red onion, carrots and mange tout and cook over a high heat for a further 2 minutes.
Give the sauce a stir and then add it to the pan along with the cooked chicken. Allow everything to simmer for a minute or until the sauce has thickened slightly. Check the seasoning.
Drain the noodles and add them to the sauce mixture. Toss everything together, then heap onto heated plates and serve.