Combine hoisin sauce, vinegar and 2 tbs hot water in a jug.
Bring a saucepan of water to the boil over a high heat. Add broccoli and cook for 2 minutes or until just tender. Drain.
Heat 1 tbs oil in a frying pan over medium heat. Add prawns and garlic and cook for 4 minutes or until cooked. Transfer to a plate.
Whisk eggs, onion (reserving 2 tbs) and sesame oil in a large jug.
Heat 1 tbs oil in a non-stick frying pan over medium heat. Pour the egg mixture into pan. Pull cooked omelette towards centre of pan allowing uncooked egg mixture to slip into pan. Place broccoli and prawns onto the half of omelette furthest from pan handle. Fold remaining omelette over filling. Place serving plate over pan and flip omelette out. Repeat with remaining oil, egg mixture and filling.
Drizzle sauce over omelettes and sprinkle with reserved spring onion and sesame seeds, to serve.