Warm the sesame oil in a wok over a medium heat. Add the onions and garlic and stir till golden brown. Add the chicken and stir fry till chicken turns white on the outside, about 6 to 8 minutes.
Add the oyster sauce and stir until the chicken is well coated. Reduce the heat to low and leave to cook for 5 minutes, stir then leave to cook for a further 10 minutes, stirring every few minutes until the chicken is no longer pink in the centre.
Finally, add the pre-cooked stir fry noodles and pepper and stir fry until hot through and everything is well mixed. Transfer to a warm plate and serve immediately.
Instead of the noodles, you could also use rice. Pre-cook the rice first and allow to cool fully before using.