Preheat the slow cooker if necessary; see the manufacturer's instructions. Mix together the minced turkey, green lentils, a little salt and pepper and the egg yolk in a bowl or food processor. Divide into 20 pieces, then shape into small balls with wetted hands.
Heat the oil in a large frying pan, add the meatballs and fry, stirring, until browned but not cooked through. Lift out of the pan with a slotted spoon and put into the slow cooker pot. Add the onion and fry until softened, then stir in the garlic, spices and ginger and cook for 1 minute.
Stir in the tomatoes, stock and a little salt and pepper and bring to the boil, stirring. Pour over the meatballs, cover with the lid and cook on low for 6–8 hours or until cooked through. Stir, then spoon on to couscous-lined plates (see below).