In a large kettle, combine potatoes, carrots, onion, parsnip, celery, lamb leg, broth, salt, and pepper. Parboil mixture over high heat. Lower the heat, place a lid to cover. Cook for 20 to 30 minutes.
When the vegetables are tender, remove from heat.
Use a colander to sieve soup into another pot.
Remove the leg of lamp.
Mash vegetables into the broth with a potato masher. Stir thoroughly.