Heat butter over a low heat, add the sliced onion and gently fry for 20 minutes
add the sugar, increase the heat for 3 minutes
reduce the heat and add the carrots, swede, garlic, tomato puree and cook for 5 minutes
add the stock, tomato sauce (or tinned tomatoes) and worcestershire sauce and simmer for 5 minutes
add the potatoes and cook for 15 minutes or until soft
add the cooked sausages and stir through