Preheat oven to 180 degrees/gas mark 4. Heat the olive oil gently in a large saucepan. Add the onion, and heat for 3-4 minutes, or until it goes slightly clear. Place all of the sausages in the saucepan and continue to move them about until they are thoroughly cooked. Transfer the contents of the saucepan into a rectangular ovenproof dish and allow to cool.