Sauté onions, until translucent, in a large sauce pan. Add cumin and curry powder (if using) to onions and let them cook for another one minute or so. Add tomatoes, other spices and lemon juice and keep stirring for a couple of minutes. Add zucchini, stir and cover the pot for a minute. Add four cups of water or broth. Bring to boil and let it simmer with sauce pan covered for about 25 minutes or until about 2 cups of water has evaporated. Add the contents to a blender and blend smooth. Serve with a dollop of fat free plain yogurt in the middle.