Put the lentils in a big pot of water and let them soak for 15 minutes or so.
Meanwhile, prepare the vegetables. Peel and dice the carrots and sweet potatoes, slice the celery and peel & slice the onion and garlic.
Heat the oil in a large saucepan, then sweat the onion, carrots and sweet potato for 8 minutes, stirring regularly.
Add the celery and the garlic then cook for 2-3 minutes. Add the passata and enough water to cover the vegetables well. Add the stock cube, and stir.
Drain the lentils and add to the soup, then add more water. (Overall you’ll need about 1.5 litre of water). Bring to the boil, then gently simmer for 20-25mins or until the lentils are tender.
Turn off the heat and stir through add the spinach leaves. Put a lid on and rest for 2 minutes.
Stir in the lemon zest, juice and fresh herbs. Season to taste.
Serve warm, garnished with fresh herbs.