Place broth, onion and curry paste in a large saucepan.
Bring to a boil. Add lentils, reduce heat and simmer covered for 5 minutes.
Add sweet potato, continue to simmer covered about 10 min until lentils and sweet potatoes are tender.
Add milk to hot soup.
Ladle about ⅓ of the soup into a blender or food processor.
Purée, then return to soup in saucepan.
Add spinach. Stir over medium heat, but don’t boil, just until spinach is wilted, 1 to 2 min.
Stir in lemon juice. Serve with a dollop of yogurt, if using.