Heat olive oil in a large pot over medium heat. Add diced onion and minced garlic, sauté until onion is translucent, about 5 minutes.
Add chopped zucchini, dried oregano, and dried thyme to the pot and cook for another 5 minutes. Pour in 4 cups vegetable broth, bring to a boil, then reduce heat and simmer for 20 minutes.
Use an immersion blender to purée the soup until smooth. If using a regular blender, allow the soup to cool slightly and blend in batches.
Return the blended soup to the pot. Stir in the lemon juice and heavy cream (if using), and season with salt and pepper to taste. Add more broth if it needs to be thinned out. Heat through.
Ladle the soup into bowls and serve hot. Garnish with fresh herbs and grated Parmesan.