Place a large frypan on medium heat, add 1 tablespoon olive oil, onion, garlic, carrots and celery and sauté for 6-8 minutes. Add lentils, tomato paste, flour, chopped tomatoes, bay leaves, herbs and 1 tin full (350-400ml) of water.
Season with sea salt and black pepper, bring to the boil, then reduce heat and simmer for 30 minutes. Whilst simmering, steam the cauliflower florets until soft, then mash or puree with remaining olive oil, sea salt and black pepper.
Preheat oven to 200C. Once lentil filling is cooked, pour into a large pie dish (1.5-2L capacity) and top with cauliflower mash. Place in top of oven and cook for 20-25 minutes. Allow to cool and set for approximately 5 minutes and serve.