Add olive oil to a soup pot over medium heat. Once the oil is heated, add the leeks and shallots and cook for until the leeks begin to soften, about 7 minutes.
Add garlic and cook for an additional 2 minutes, and then stir in the butter, celery, potatoes, parsley, thyme, salt, and pepper.
Pour in the vegetable broth and lemon juice and bring the soup to a boil, then turn the heat down to medium low, cover, and let the soup simmer for 25 minutes.
Use a high speed, heat safe blender or an immersion blender to purée the soup until it's smooth and creamy.