Heat half the olive oil in a casserole dish or oven-proof saucepan over a low to medium heat, add the sliced onions and cook for 15 minutes, stirring frequently. Then, remove the onions from the pot with a slotted spoon and set to one side.
Add the remaining oil to the pot and heat for 1-2 minutes. Stir-fry the beef, a little at a time, so not to overcrowd the pot.
Once the beef is nicely browned, return the fried onions to the pot along with the garlic, turmeric and cumin. Stir for 2 minutes.
Stir in the tomatoes, tomato purée and hot beef stock. Bring the pot to a simmer, place on the lid and transfer to the oven to cook for 1 hour.
Remove the pot from the oven and stir in the washed lentils, nutmeg and cinnamon.
Reduce the oven temperature to 150°C/gas mark 3/300°F and cook for 1 ¼ –1 ½ hours, until the meat is very tender.
Finish the dish by stirring in the fresh lime juice and chopped mint, adjust the seasoning to taste with a little salt.