Choose your favourite veggie/vegan burger (make sure it is no more than 200 calories) and cook according to pack instructions. Meanwhile, half an avocado and mash with a squeeze of lemon or lime juice and a some finely chopped chilli (optional). Very lightly toast a 60g wholemeal roll. On the bottom part of the roll spread the avocado and lay on some lettuce. Put on the burger on and top with slices of tomatoes and onion rings. Finish with the other part of the roll and serve.