Preheat the oven to 180°C/350°F/Gas mark 4.
Remove any excess fat from the lamb. Skin and core the kidneys, and cut them into slices.
Arrange a layer of potatoes in the bottom of a 1.8-litre/3pt ovenproof dish.Arrange the lamb neck cutlets on top of the potatoes, and cover with the kidneys, onion and thyme. Pour the stock over the meat, and season with salt and pepper. Layer the remaining potato slices on top, overlapping to completely cover the meat and onion.
Brush the potato slices with the butter, cover the dish and cook in the oven for 1 1⁄2 hours. Remove the lid and cook for a further 30 minutes until golden brown on top. Serve hot.