Check the split peas for stones and remove.
Rinse peas.
Place peas in a bowl and fill with water.
Soak for a few hours.
Once peas have finished soaking, chop the onions and celery.
Slice the carrots, leeks and cube the potatoes.
Put all of the ingredients (except crackers and garnish ingredients) in a 10-quart heavy bottom pot, and bring to a boil for 5 minutes.
Reduce heat to medium-low and cover loosely.
If necessary, add water to keep the vegetables submerged.
Cook for at least 1 to 1 ½ hours or until veggies are fork tender.
Check seasoning.
Add salt, pepper or seasonings as desired.
Purée the soup with an immersion blender until completely done.
To serve, ladle soup into bowls.
Garnish soup with salt, pepper, and parsley or thyme.
Serve with oyster crackers or bread, if desired.