Trim the very top and bottom off the leek. Halve it and rinse out any grit. Finely slice the leek. Warm a large pan for 1 min. Add 1 tbsp oil and the leek. Season with salt and pepper. Cover and turn the heat down low. Sweat for 10 mins till the leek is soft, stirring occasionally.
While the leek sweats, finely chop the courgettes. Peel and grate or finely chop the garlic cloves, ginger and turmeric. Finely chop the stalks from the coriander (keep the leaves aside for later).
Stir the courgettes into the leek. Add the garlic, ginger, turmeric, coriander stalks and half the coriander leaves. Crumble in the stock cube and pour in 800ml boiling water (or use hot homemade stock). Cover, turn the heat down and simmer for 10 mins.
Add the baby leaf spinach to the pan and stir for 2 mins to wilt the leaves. Ladle into a blender and blitz till smooth, or use a hand-held blender in the pan. Taste the soup and add more salt and pepper if you think it needs it. Ladle the soup into warm bowls and top with the coriander leaves.