Mix the ingredients for the teriyaki sauce and set aside.
Slice the spring onions at a 45° angle.
Peel the garlic cloves and dice them as finely as you can.
Cut the ginger into batons around 25mm (1") long and as thinly as you can.
Heat a 25cm or 10" frying pan over a high heat and add the oil when the pan is shimmering, then throw in the prawns and stir fry for 90 seconds.
Add the garlic, ginger and chilli and stir fry for 30-60 seconds.
Finally, add the prepared sauce and cook for 2 final minutes. If the sauce has not thickened, remove the prawns and cook over a high heat for another minute or so.
Serve sprinkled with the spring onion and toasted sesame seeds.