- Heat oil in large saucepan over medium heat.
- Add leeks and 1 tbsp water, and cook, stirring often, until tender, about 5 minutes.
- Add curry powder, and stir 30 seconds.
- Add stock, potatoes and salt, and bring to a boil.
- Reduce heat to low, cover and cook until potatoes are tender, about 10 minutes.
- Stir in spinach; cook until just wilted, about 1 minute.
- Cool slightly.
- Purée soup in blender until almost smooth; work in batches if necessary.
- Return soup to saucepan.
- Add milk, and cook over medium-low heat.
- Season to taste with salt and pepper.
- Ladle soup into 4 bowls.
- Garnish with swirl of yogurt, if desired, and serve.