Heat up all 12 cups of water in a large pot. Do not fully boil. The temperature of the water should reach about 150 degrees F (65 degrees C), but not more than that.
Remove the pot from the fire. Pour the water into a bowl and add all of the sumac berries into it. Mix the berries so all of them get submerged under water. Let the berries soak in the water for at least 30 minutes. This will color the water red and absorb the tart flavor of the sumac into the water.
While the sumac is soaking, chop the onion into small pieces and saute it in a frying pan with olive oil until the onions become golden brown, for at least 15 minutes, while constantly stirring the onions. If there is too much olive oil left in the pan when onions are done, drain it out of the pan.
Strain the berries out of the water by pouring it over a strainer, back into the pot where the water was heated up. Strain the water twice, to make sure that all the berries have been removed. If there is some sumac sediment residue left in the water, it is ok, and it does not need to be removed. The sumac berries can be thrown away, as they will not be used anymore.
Add all of the lentils to the water and bring to a boil. Make sure the pot is covered with the lid to make everything boil faster.
Simmer the soup for 30 minutes. Stir the pot every 5 minutes. While the pot is not being stirred keep the lid on.
After the soup has simmered for 30 minutes, taste the lentils to make sure they are fully cooked.
Add all of the sauteed onions, salt, pepper, and coriander to the soup. Mix it well and taste. If needed adjust the amount of the seasoning.
Cook the soup for another 30 minutes with the lid off to boil off excess water, while mixing the soup every 5 minutes.
Pour all of the soup into a blender or food processor and run it for about 30 seconds until the soup is completely smooth. Serve.
The taste of the soup will be a little spicy due to the spices and sour due to the sumac berries.