Heat olive oil in a large heavy saucepan over medium heat until the foam subsides.
Add leeks, potatoes, curry powder, salt, and pepper and cook, stirring occasionally, until leeks are softened, about 5 minutes.
Stir in water, herbs, and stock, bring to a simmer, and simmer, uncovered, until potatoes are very tender about 5 minutes.
Transfer 1 cup of soup to a blender (use caution when blending hot liquids) and puree until smooth.
Return puree to the pan, stir in water to desired thickness and bring to a simmer.
Stir in spinach and simmer until spinach is wilted, about 1 minute more.