Add a little oil to a large oven-proof dish and cook the sausages (this can be done in 2 batches) on the hob until well-browned on the outside. Place to the side.
Again using the oven-proof dish, add the sliced onions, leeks, garlic cloves and cook on the hob for a few minutes to caramelise. Return the sausages to the dish, along with the bay leaves and a good pinch of salt & black pepper.
Cover with 1.5litres of stock and place in the oven (180c / 160c fan) and cook for approx 1 hour until the leeks are well-cooked.
Meanwhile, peel and quarter the potatoes. In a large pan, boil in salted water until tender and then drain.
Allow the potatoes to steam in the colander for a few minutes before returning to the pan.
Add 3 tbsp of salted butter and a pinch of black pepper to the potatoes before mashing thoroughly. Keep warm on a very low heat until ready to serve being careful not to burn the bottom of the pan.
Serve the buttery mash with the sausages casserole and your favourite vegetables.