Rinse and drain peas and add to pot with water, broth, onion, garlic, lemon juice, salt, parsley, white pepper, and thyme.
Bring to boil, stirring occasionally.
Reduce to medium and cook 1 hour and 30 minutes, stirring occasionally.
Rinse barley and cook according to directions on package.
Once soup has thickened, remove 1/4 of soup and set aside.
Carefully blend the soup in pot using a hand blender until pureed.
Drain barley and add to soup.
Return the 1/4 of soup to the pot and add carrots and celery.
Simmer soup for about 15 minutes until carrots are slightly soft but still somewhat crisp.
Turn off heat, cover, and let soup sit for 15 minutes.
Garnish with fresh parsley or spring onions if desired.