Cut off the upper dark green part of the leeks and discard. You may use the very light green part of the leeks if desired. Cut off any of the root end if necessary and discard. Cut the leek in half lengthwise, then cut again lengthwise. Now cut the leeks into small chunks. Place in a large bowl and rinse in cold water to remove any possible dirt. Strain through a sieve.
In a medium size heavy bottom cooking pot, melt butter on low heat. Add chopped leeks and cook for 10 minutes until slightly softened. Add stock, water, chicken bouillon cubes, cubed potatoes, and bring to boil; reduce heat and simmer for 10 minutes.
Add peas, chopped mint, dill weed, salt, white pepper and cook for another 5 minutes or until the vegetables are tender soft.
Puree soup with an immersion blender or puree in a food processor, then return to the cooking pot and reheat if necessary. Adjust seasonings to taste.
When serving, add 1 dollop of sour cream or yoghurt or swirl into the soup in each bowl served.