Heat the oil in a frying pan and soften the onion. Add the rinsed lentils and water. Bring to the boil, then simmer for 20 minutes or so until soft.
Drain the lentils then put them back to the pan. Add the rinsed spinach leaves. Cover the pan with a lid, leave for about 5 minutes until the spinach has just wilted.
Transfer it to a serving dish. Add the yoghurt, oil, parsley and tomato slices. Season to taste with salt and pepper, then squeeze in the juice of the lime/lemon. Toss everything together and serve.