Heat 1 tablespoon oil and 20g butter in a frying pan over medium–high heat. Add 6 prawns and one-quarter of the onion, and cook, tossing occasionally, for 2 1/2 minutes or until prawns are pink. Using a ladle, add one-quarter of the egg mixture. Season with salt and pepper. Cook for 5 minutes or until omelette is set and prawns are just cooked through. Transfer omelette to a plate and cover loosely with foil to keep warm. Repeat 3 more times with remaining oil, butter, prawns, onion and egg mixture.