Wash string beans and chop into 1cm pieces. Bring 2-3 cups of water to boil. Add chopped string beans and a bit of salt. Cook over medium heat for about 10-15 minutes under tender. Drain water.
Peel and chop onions. Fry in oil until slightly golden. Dissolve saffron in 1/3 cup of hot water. Beat eggs well, then add baking soda, salt, black pepper, saffron, flour, onions and string beans. Mix well.
Heat oil in a non-stick pan until it is hot. Pour in the mix, flatten the surface with the back of a spoon, and place the lid on. Reduce heat and fry for about 10 minutes, until kookoo is cooked under. Cut radially into 4 equal pieces, turn over, and fry for another 10 minutes.