Melt half the butter with the oil in a sauté pan or flameproof casserole and fry the sausages with the mushrooms and onion until lightly browned.
Add the lentils, stock, oregano and tomato paste and mix the ingredients together. Bring to the boil and cover with a lid, then reduce the heat and cook very gently for about 20 minutes until the lentils are tender and the stock is nearly absorbed.
Stir in the tomatoes and check the seasoning. Cook for a further 5 minutes.
Meanwhile, mix the garlic and parsley with the remaining butter and spread thinly over the ciabatta slices. Arrange over the hotpot and cook under a preheated moderate grill for about 5 minutes until the bread is lightly toasted.