Make the marinade by adding the garlic paste, ginger paste, soy sauce, sesame oil and white pepper to a small bowl then mix the ingredients together. Add the chicken breast to the marinade and fully coat the chicken in the marinade. Cover the bowl and refrigerate for 1 hour.
Line a baking tray with parchment paper and preheat the oven to gas mark 6/400oF/200oC/180oC fan. Place the marinated chicken breasts along with the marinade on the baking tray and bake in the oven for 20 - 25 minutes. Remove from the oven, allow the chicken to cool then chop it into small pieces and set aside.
To make the chow mein sauce, add the oyster sauce, soy sauce, light brown sugar and sesame oil to a small bowl then mix and set aside.
Place a pot of water on a high heat and bring to a boil. Once the water is boiling add the noodles and cook or soak according to the package instructions. Strain the noodles, rinse with cold water then set aside.
To a large frying pan, add the sunflower oil and allow to heat on a high heat. Once the oil is slightly sizzling, add the sliced onions and stir fry for around 2 minutes. Add the noodles and chicken with any juices then continue to stir fry for a further 1 minute. Add in the beansprouts then pour in the chow mein sauce and combine well.
Remove from the heat and serve. Optionally you garnish with chopped spring onions.