Heat butter in a large saucepan over low-medium heat, add onion and garlic and sauté until tender (10-12 minutes). Add ham hocks, cover with cold water (about 4 litres), bring to the simmer, reduce heat to low and cook until ham is falling from the bone (1-1½ hours). Remove hocks from saucepan, set aside to cool and, when cool enough to handle, remove skin and bone (discard), coarsely shred meat and set aside.
Meanwhile, add split green peas to stock and simmer until very tender (45 minutes-1 hour). Add baby peas (reserve some for garnish if desired) and simmer until bright green (1-2 minutes). Purée in batches in a blender until smooth, add shredded ham to soup (reserve a little for garnish if desired), season to taste and keep warm.
Meanwhile, for herb butter, combine ingredients in a small bowl, season to taste and set aside.
Preheat a grill to high. Place baguette cut-side up on a baking tray and grill until golden (1-2 minutes each side), then spread with herb butter. Serve with hot soup topped with reserved baby peas (steamed until bright green) and shredded ham.