How to Make It
In a large skillet, heat the vegetable oil. Season the lamb chops with salt, pepper and cumin. Add the lamb to the skillet and cook over moderately high heat until browned, about 3 minutes per side. Transfer the lamb to a large plate.
Preheat the oven to 350°. Melt the butter in the skillet. Add the onions, season with salt and pepper and cook over moderate heat, stirring occasionally, until starting to soften, about 10 minutes. Reduce the heat to low and cook the onions until very tender, about 5 minutes. Sprinkle the onions with the turmeric and thyme and cook, stirring, until fragrant, 2 minutes. Scrape the onions into a bowl. Add the beef stock to the skillet and boil, scraping up the browned bits from the bottom, about 1 minute.
Butter a 9-by-13-by-2 inch glass or ceramic baking dish. Spread half of the potatoes in the baking dish and season generously with salt and pepper. Spoon one-third of the onions over the potatoes and arrange the lamb chops on top. Cover with one-third of the onions and then the remaining potatoes; generously season the potatoes with salt and pepper. Top with the remaining onions. Pour the lager over the onions and then pour in the beef stock from the skillet.
Cover the dish with foil and bake in the upper third of the oven for 1 hour. Uncover the baking dish, increase the oven temperature to 400º and bake for about 30 minutes longer, until the potatoes are tender and the top is browned. Let the dish rest for 15 minutes before serving.