Put water into a pot then add the onions, garlic, celery and carrots. Cook gently for 5-7 minutes until soft. Meanwhile add the spaghetti to a pot and cook as per the packet instructions.
Add lentils, stock and can of chopped tomatoes. Bring to a boil then turn down and simmer for 20 minutes until the lentils are cooked.
At the last minute before serving, stir in a handful of spinach leaves.
Drain the spaghetti and then serve on to warm bowls. Spoon the lentil bolognese sauce over the spaghetti and grind the fresh pepper over the sauce.