Heat olive oil in a large pot over medium heat. Add onion and cook until softened, about 5 minutes.

Add zucchini and cook for another 5 minutes, until slightly tender.

Pour in vegetable stock and bring to a boil. Reduce heat and simmer for 10 minutes.

Remove from heat and let cool slightly.

Carefully transfer the soup to a blender (or use an immersion blender) and blend until smooth.

Stir in mint leaves and lemon juice. Season with salt and pepper to taste.

Serve immediately or chill for later.