Preheat the oven to 180°C/ Gas mark 4. In a large casserole dish, heat the olive oil and fry the sausages, so they are browned on all sides. Remove from the dish and set aside.
Add the chopped onion, celery and carrots and fry for 3-4 mins until softened. Add the stock, chopped tomatoes and baked beans and stir well.
Return the sausages to the dish, add the bay leaves and season well. Bring back to the boil, then layer the potato slices over the top of the casserole dish.
Brush the top with the melted butter, cover with a lid and put in the oven for 40 mins.
After 40 mins, turn up the heat to 200°C/ gas mark 6, remove the lid and cook for a further 15-20 mins or until the potato is brown and crisp.