Heat the oil in a large soup pot.
Sautee the onions and garlic for about 5 minutes, until the onions are softened.
Add the garlic and tomato paste, and continue to cook for another minute or two.
Pour in the vegetable stock, and stir in the cumin, cayenne pepper, and curry powder. Add the lentils.
Cover and simmer for 15 to 20 minutes, until the lentils are soft.
If you like a smooth texture, you can puree the soup at this point with a blender or food processor. If you do, return it to the pot for the next step.
Add the lemon juice and zest, and salt and pepper to taste, stirring to blend in the flavors.
Now add the spinach, stirring until the leaves are wilted and blended into the soup.
Sprinkle with parsley, and serve with a dollop of plain yogurt on top