Heat 1 tablespoon of olive oil in a medium-sized pot over medium heat.
Add the onion and garlic to the pot. Sauté until they become fragrant and translucent.
Stir in the celery and thyme, and cook for an additional minute.
Pour in the vegetable stock and bring the mixture to a boil.
Once boiling, add the diced potatoes and cook until they are tender, about 15 minutes.
Add the watercress to the pot and boil for 2 more minutes.
Carefully transfer the soup mixture to a blender. Blend until it becomes a smooth and creamy soup. *You can leave the soup in the pot and use an immersion/ stick blender to blend the soup.Taste and adjust the seasoning and add in the lemon juice. *Start with the juice from ½ the lemon and add more if desired. In a separate pan, fry the diced potato for garnish until golden and cooked through. *This is optional.
Serve the soup in bowls, topped with fried potato dices, croutons, and a dollop of lemon pepper vegan yogurt. Drizzle with a bit of olive oil for an extra touch.