Arrange half the meat in the bottom of a medium, flameproof casserole and cover with half the onions followed by half the potatoes, seasoning each layer with salt and pepper and a little thyme. Repeat the layers, seasoning as before and finishing with potatoes. Pour over enough water to just cover the top layer.
Bring to the boil, reduce the heat, cover and cook very gently for 1 1/2 hours, or until the meat and potatoes are tender. Check frequently to make sure that the water has not evaporated and add a little more, if necessary. Serve with the traditional accompaniment of steamed red cabbage, if desired.