Boil the water and keep aside.
Set a large pot/casserole over medium heat and let it heat up for 1-2’. Add the oil let this heat up for 1’ more. Add the onions along with the carrots, celery, garlic, leeks and chilli flakes and sauté for 7’ until softened, not browned. Add the potatoes, courgettes, garlic, bay leaves, thyme and oregano and sauté for 5’ more adding more oil if needed.
With the heat set to high add to the pot/casserole the celery and parsley leaves along with the water, just enough to cover the veggies plus 1cm above, and bring to a boil. Turn the heat to low and simmer, covered, for 20’ until the potatoes are soft. Check doneness with a fork. Season with the salt and pepper and give it a final stir.
Take the pot/casserole off the heat, remove and discard the bay leaves and with a handheld blender blend until smooth.
Serve in individual bowls, sprinkle some sumac while drizzling with olive oil.