Heat a non–stick frying pan. Add sausages and gently fry gently until just browned. Remove from the pan and slice on the diagonal, lengthways into three. Meanwhile heat a large frying pan. Add the 55g / 2oz of the butter and once melted add the onions. Cook the onions gently for 10 minutes or until soft and translucent. Remove from the pan and set aside. Add the bacon lardons to the frying pan and cook for 3 – 4 minutes until golden and crispy. Add the Cavalo Nero with 2 tablespoons of water. Cook for 2 – 3 minutes or until the cabbage is soft, but still holding a little texture. Drain off any fat, remove from the pan and set aside.
Rub a large gratin dish (30 x 21 x 6cms) liberally with 25g / 1oz butter. Place a layer of sliced potatoes in the bottom of the dish season with salt and pepper and a sprinkling of the thyme. Top with a layer of sliced sausage, then a layer of cooked onion. Top with another layer of sliced potato and season with salt, pepper and thyme. Spoon over the cooked Cavalo Nero and bacon. Top with another layer of potato slices and season again with salt, pepper and thyme. Top with a layer of sliced sausage, then a layer of cooked onion and finally a layer of sliced potato. Season once again with salt, pepper and thyme.
Pour the cream and stock into a pan and add the bay leaves. Place over a gently heat and bring to just below boiling point. Allow to cool slightly, then pour over the egg yolks and whisk to combine. Pour over the sausage and potato pie. You want the liquid to just cover the top layer of potatoes. Dot over the remaining butter and cover with foil. Place the dish on a baking sheet. Place in the oven and cook for 1 ¾ hours. Remove the foil from the dish and increase the oven temperature to Gas 6 / 200c and cook for a further 15 minutes or until the top is golden. Serve immediately with garlic bread.