Heat the oil in a large pan over medium heat. Once the oil is simmering, add the onion, carrot, zucchini and cook, stirring often, until softened.
Add the garlic and spices and cook until fragrant, stirring often. Pour in the lentils, tomatoes, stock and water. Add chilli flakes (optional) and season with ground black pepper.
Turn up the heat and bring to the boil, then partially cover the pot with a lid and reduce heat to maintain a gentle simmer. Cook for 30 minutes, or until lentils are tender but still hold their shape.
Add the spinach leaves and cook for 1-2 minutes until wilted. Season with lemon juice and pepper. Serve with wholegrain bread.