Soak the peas in water for at least 90 minutes, or overnight. Change the water a few times.
Take the whole onion and nail the bay leaves into it with the whole cloves.
Add the oil to a soup pot. When hot, add the diced onion and chopped leek and saute until they're soft. Add the chopped carrots, potatoes and celeriac and continue to saute for a few minutes.
Add water until the vegetables are just covered.
Rinse the peas an additional time and add to the pot with the vegetables. Add the whole studded onion and the ham bone. Wrap the herbs and tie with kitchen twine, and add it to the pot as well (leaving a little string to pull it out easily).
Add more water to cover and cook over medium (not hot) heat,slowly, until it boils. Remove the skim as needed as it comes to a boil.
When boiling, lower heat to a simmer, and cook at least two hours uncovered.
Remove the tied herb bunch, the whole studded onion and the ham bone.
Using a stick blender, puree the soup until well blended.
Taste before salting, as the ham is pretty salty. Top with cracked pepper, and any ham bits that were cut from the removed bone.