In a large saucepan over medium heat, warm the oil and saute the onion and celery with 1/4 teaspoon of the salt for about 5 minutes, until translucent. Add the zucchini, 1/4 teaspoon salt, and saute for a further 3 minutes.

Add the vegetable broth and the remaining 1/2 teaspoon salt, and stir in the almond butter until well combined. Increase the heat to high, and bring to a boil. Reduce the heat to low, and simmer for about 5 minutes, until the zucchini is tender. Add the watercress and simmer for a further 3 minutes, then turn off the heat, and allow the soup to cool slightly. Stir in the lemon juice.

Pour the soup into your blender in batches and puree on high for 30 to 60 seconds, until smooth and creamy. (For conventional blenders, remove the small center lid cap and cover the opening with a kitchen towel so steam can escape while you blend.) Return the soup to the saucepan, season to taste, and warm it over low heat.

To serve, ladle the soup into bowls and drizzle with olive oil. Pass lemon juice at the table.