Prepare lentils and place in pan with water to cover, and bring slowly to the boil.
Cook gently until soft, about 3/4 hour, adding salt, just before done.
Peel and slice onion and fry in butter until browned.
Season and beat into lentils, adding half the grated cheese.
Place in a greased casserole and sprinkle with the rest of the cheese.
Dot with butter and bake in a hot oven until browned.