Peel and dice the onion.
Heat sunflower oil in a pot, add onions and fry until glassy.
Add tomato paste, flour, all spices and mix everything well.
Peel and dice potatoes.
Pull off rhubarb fibers, to do this, place a kitchen knife at the bottom of the stem and pull off the hard fibers towards the top. Peel the rhubarb in this way until no more strings come off. Cut into slices.
Add the potatoes and rhubarb to the onions.
Now add boiling water until the soup is covered. Simmer for 25 minutes on medium heat until rhubarb is dissolved and potatoes are cooked.
Crack eggs into a bowl, add salt, pepper and whisk.
Pour the whisked eggs into the boiling soup and stir gently.
Serve in bowls and drizzle with oil.
In case the soup is too sour for you, stir in some yogurt.
The rhubarb soup is eaten with dry bread, preferably with traditional Persian bread, such as dry lavash bread.