Preheat the oven to 180°C.
Butterfly the prawns horizontally while keeping the shells intact and removing the digestive cords.
Rinse the prawns in cold water and pat them dry with kitchen paper.
Stir-fry 30 ml of olive oil with garlic, onion, spring onions, and chillies for 2 minutes.
Season with salt and pepper, then transfer the mixture to a bowl.
Fry the prawns in the remaining 45 ml of olive oil for 30 seconds on each side.
Place prawns on a baking tray and stuff them with the onion mixture.
Roast the prawns in the oven for 3 to 4 minutes until cooked.
Simmer the remaining stuffing with butter in a saucepan for 3 minutes to make the sauce.
Serve the prawns on a platter with lemon wedges and the butter sauce.