The key ingredient in Kuurdak is lamb fat (tail fat), which is slowly rendered and then used to fry all other ingredients. If you don't have lamb fat, you can use butter or vegetable oil, but the fat adds an authentic and rich flavor.
The meat and potatoes should be browned and crispy, and the onion caramelized.
In a heavy-bottomed cauldron (kazan) or large pan, render the diced lamb fat over medium heat. You can remove the resulting 'cracklings' for use in another dish or leave them in.
Increase the heat. Add the lamb chunks and fry, stirring occasionally, until dark golden brown on all sides (about 10-15 minutes).
Add the onion. Fry until softened and starting to caramelize (about 5-7 minutes).
Add the diced potatoes, salt, black pepper, and cumin. Stir and fry the potatoes for about 10 minutes to coat them with fat and start browning them on all sides.
Add the hot water (about 100 ml). The amount depends on how thick you want the sauce; it should not be more than a quarter of the dish's volume.
Cover and simmer over low heat for 20 minutes until the potatoes are completely tender. Add a little more water if necessary.
Serve immediately, hot. Kuurdak is served directly from the kazan or in one large bowl, sprinkled with fresh herbs.