Heat the oil in a large, heavy-based saucepan over a medium-low heat, add the onion, ginger and garlic and cook gently for 10 minutes until softened but not coloured.
Stir the lentils and cinnamon into the onions. Add the tomatoes and measurement water or stock. Season with salt and pepper and bring to the boil. Reduce the heat, cover and leave to simmer gently for 30–35 minutes or until the lentils are tender and the liquid has been absorbed.
Spoon the lentils into bowls and sprinkle with the black onion seeds and garnish with parsley leaves. Serve warm with lemon wedges and toasted wholemeal flatbreads, if liked.