Cut zucchini in half lengthwise, and if using larger zucchini, remove the seeds and cut into chunks. Brush with olive oil, reserving 2 Tbsp. olive oil for the soup. Sprinkle with salt and pepper.
Roast zucchini for 25-30 minutes, or until tender and starting to brown. (smaller zucchini may take less time.) Let zucchini cool, then remove the skin (it should peel right off. If some pieces of skin are stubborn, don't worry about it.
In a large pot, heat the remaining 2 Tbsp. olive oil and add the onion and garlic. Cook over medium heat, stirring frequently, until the onions are translucent. Add the spices, and cook for a minute more until the spices are fragrant.
Add the chicken broth and zucchini, reduce heat to low, and simmer 25-30 minutes.
Puree the soup in a blender or using an immersion blender, and chill. Once cold, add the buttermilk.
Serve with herbs and/or croutons for garnish.
Recipe by The Domestic Front at https://www.thedomesticfront.com/cold-zucchini-soup-buttermilk-much-zucchini-part-iii/