In a frying pan over a medium heat, warm a little oil then add the onions and sausage then cook until the onions are soft and the sausage browned.
In a small saucepan over a high heat, combine the stock cube and water then add the carrot, celery and tomato puree. Bring to the boil then add the sausage and onions, simmer until the carrots are almost cooked, about 8 to 10 minutes.
Preheat the oven to 180 degrees C.
Put the mix into an ovenproof dish and arrange the sliced potato on top. Brush with a little vegetable oil and finish with a sprinkle of salt.
Bake in the oven for 20 to 30 minutes or until the potato is tender and golden brown. Remove from the oven and serve.