Preheat a slow cooker or a large cast-iron saucepan over a medium heat and sauté the leeks, carrots, garlic and ginger in a generous glug of olive oil for 5 minutes or until lightly browned
Add the spices and stir, cooking for 1 minute
Stir in the lentils, tomatoes and vegetable stock
Bring to a boil, then reduce the heat to medium-low, keeping the soup at a gentle simmer
Cook, stirring every now and then, for 25-30 minutes or until the soup has thickened and the lentils have broken down
Season well and stir in the lemon juice and zest, as well as the parsley. Serve immediately.