In a medium/large casserole pot, heat the avocado oil over a medium heat then reduce to low heat and add onion and garlic. Cooking on a lower heat avoids the garlic and onion browning. Cook until the onion is translucent, about 5 minutes.
Add courgettes and 500ml water (or stock) and bring to the boil. Reduce heat and simmer for 10 minutes or until courgettes are cooked.
Stir in spinach and basil, and puree using hand immersion blender (or pour into a jug blender). Puree well until all the ingredients are combined. Add remaining water to reach desired consistency. Season with salt and pepper and add a few sprigs of parsley before serving.
This amount makes 1.5 litres. Keeps for five days in the fridge and freezes well.