Heat the oil in a wide pan on the stove. Peel and chop the onion, cooking it over a medium heat until it begins to soften.
Add the mint, oregano and crushed chilli flakes. Stir over the heat, then break up the ground lamb in the pan with a wooden fork or spoon until it begins to lose its pinkness.
Stir in the red currant jelly and Worcestershire sauce, and then take off the heat and tip into your slow-cooker.
Tip the chopped tomatoes straight into the bowl then divide the 2 cups of water between the empty cans to swill out any remaining tomato-y juices. Pour into the slow-cooker pot, stirring briefly.