Prepare the vegetables - peel and finely slice the onion, wash and finely slice the leek into rounds, peel the carrots then cut into ribbons with the vegetable peeler.
Cook the sausages according to the pack instructions.
Meanwhile, heat the oil in a frying pan. Fry the onions on a low heat for 2-3 minutes until starting to soften.
Add the leeks and fry for a further 2-3 minutes.
Add the carrot ribbons and fry for another 2-3 minutes.
Turn up the heat to medium then add 1/4 cup of cold water to the vegetables.
Stirring occasionally, cook for 10 minutes on a medium to low heat until all the water has been absorbed and the vegetables are soft.
Split the bread rolls down the middle and fill with the rainbow vegetables.
Top each roll with a cooked sausage, then serve with ketchup and mustard on the side.