In a large non-stick frying pan over a low medium heat cook the sausages, turning every 10 minutes until golden all over and cooked through, 30 to 40 minutes.
Remove from the pan, cover with foil and set aside. Into pan juices, add onions, cooking until golden.
Sprinkle with sugar and toss to coat, lowering heat to caramelize, 5 minutes.
To serve place sausages then top with the caramelised onions and tomatoes.