Heat 2 tablespoons of olive oil in a large pot over medium heat. Add the onion, celery, and carrot and cook until softened, about 5-7 minutes.
Add the broccoli stems and potatoes and cook for another 3-5 minutes.
Pour in the ouzo and cook for 1 minute, allowing the alcohol to evaporate.
Add the vegetable broth and star anise. Bring to a boil, then reduce heat and simmer for 15-20 minutes, or until the potatoes are tender.
Remove the star anise. Add the broccoli florets and cook for another 5 minutes, or until tender-crisp.
Remove from heat and carefully blend the soup with an immersion blender or in a regular blender until smooth.
Stir in the Greek yogurt and lemon juice. Season with salt and pepper to taste.
Serve hot, garnished with pumpkin seeds and a drizzle of olive oil.