In a large pot, heat oil and cook onion until soft (approx. 6-7 minutes). Add garlic, carrots and celery and cook on medium heat for 5 minutes, stirring frequently.
Add crushed tomatoes, tomato paste and vegetable stock and bring to a gentle simmer for 20 minutes.
Add lentils, season with salt and pepper and cook for a further 10 minutes.
Garnish with parsley and sliced chilli or capsicum (optional) and serve.