Heat the oil in a frying pan then gently fry the onion over a medium heat until golden. Add the garlic and cook for a further 2–3 minutes until soft but not browned. Then add the tomato purée, tinned tomatoes and 200ml of water and stir well.
Transfer this mixture to a blender, add the Medjool dates and purée until completely smooth. (If you don’t have a high-speed blender you might want to soak your dates in warm water for about an hour beforehand.)
Pour the mixture into a deep oven dish and add the remaining ingredients. Give it all a good stir to completely combine.
Bake in the oven for about 60 minutes. Stir in another 200ml of water and then bake for another 60 minutes (2 hours in total).
Remove from the oven and leave to cool slightly before serving. If the sauce is a little thick you can add more water.