Heat oil in a large saucepan and fry onion, garlic and coriander stems over low heattill soft and golden coloured, about 10 minutes.
Add curry paste, stir to mix and cook for 2 minutes. Add butternut squash, chickpeas, coconut milk and water. Bring to the boil, then reduce heat and simmer, covered, for 35 to 40 minutes, stirring every so often. Add a little more water if dry.
Remove lid and simmer for a few minutes longer so the sauce thickens.
Add spinach and half of the coriander leaves. Stir to mix. Cook for 1 further minute till the leaves have wilted. Remove from the hob.
Serve sprinkled with the remaining coriander leaves.