Add the onion, split peas, broth, and water. Drop in the celery seed, carrots, salt, and pepper.
Cover and cook on low for 8-10 hours or until the onion is translucent and the peas are soft.
Carefully remove the ham bone.
Pulse the soup with a hand- held immersion blender (or let it cool and blend in small batches in a traditional blender) until you’ve reached your desired consistency.
This soup freezes quite well, but you may need to add a bit of broth upon reheating.