Freeze the lamb fillet for 30 minutes before slicing to make it easier to slice against the grain thinly
Using a sharp knife, slice lamb as thin as you can at an angle
Add the lamb to a bowl with the marinade ingredients (soy sauce, water, vinegar, cornflour). Mix well and marinate in the fridge for a minimum of 30 minutes
Make the sauce by combining the oyster sauce, soy sauce, white pepper, sesame oil and brown sugar. Whisk well and set aside
In a large wok heat 2 tablespoons of vegetable oil and seal the lamb in batches on high heat, just enough to have some colour without fully cooking it. Set aside
Heat some more oil in the same pan and cook off the garlic and ginger until fragrant. If using white onion add it now and cook for a few minutes
Add the lamb back in along with the sauce. If using spring onion stems add them now
Stir through and simmer for 5 minutes until sauce thickens slightly
