Preheat the oven to 180ºC.
Place the whole sweet potatoes on a try, prick it with a fork and drizzle with a tablespoon of olive oil. Place a whole head of garlic beside the sweet potato.
Roast for 40-50 minutes until the potato is soft and the garlic is caramelized. Let them cool down a bit and peel the potato.
To make the soup, heat the other tablespoon of oil in a pot over a medium heat. Sautée the onion and celery for about 15 minutes.
Add the roasted sweet potato, two cloves of roasted garlic (you can use the rest for salad dressings or as butter for garlic bread), the sumac and the water. Bring to a simmer, and cook for 20 minutes.
Blend in a food processor or with a hand blender. Check seasoning.
Serve with a spoonful of yogurt and a dusting of sumac