Place the sausages on a greased baking tray and cook according to the instructions on the packet, turning once or twice during cooking. Slice in half lengthways when cooked.
Meanwhile, heat the oil in a large frying pan. Add the onions and cook gently for 20 minutes until soft and brown. Add the sugar and vinegar and cook for another 5 minutes.
Spread the chutney on 4 slices of bread. Place the sliced sausages on top, then cover with the onions, tomato and rocket. Top with the remaining slices of bread and serve immediately.