Start by washing the zucchinis thoroughly. Cut off the ends, and then slice each zucchini into rounds.
Peel and chop the onion and garlic cloves finely.
In a large pot, heat the olive oil over medium heat. Add the chopped onion and garlic, cooking until the onion becomes translucent (about 5 minutes).
Add the sliced zucchinis to the pot and stir well. Cook for another 5 minutes, allowing the zucchinis to soften slightly.
Pour in the vegetable broth and bring the mixture to a boil. Reduce heat and let it simmer for about 15 minutes, or until the zucchinis are completely tender.
Remove the pot from heat and add the fresh basil leaves. Using a blender or immersion blender, puree the soup until smooth.
Season with salt and black pepper to taste. If desired, reheat the soup briefly before serving.
Ladle the soup into bowls and garnish with grated Parmesan cheese, if using.
For a creamier texture, you can add ½ cup of coconut milk or heavy cream after blending.