Preheat oven to 220C. Place lamb bones in a roasting pan and roast until golden (10 minutes).
Meanwhile, heat olive oil in a large saucepan over medium-high heat, add lamb shank and cook, turning occasionally, until golden (4-5 minutes). Reduce heat to medium, add onion and garlic, sauté until softened (4-5 minutes), add fennel bulb and potato and cook until tender (4-5 minutes). Add lamb bones and 3 litres of water, bring to the boil and simmer until meat falls from the bone (2 hours). Remove bones and shank from liquid, shred meat with a fork (discard bones) and return to pan. Bring to the simmer, add peas and cook until peas are tender (1-2 minutes). Season to taste, scatter over reserved fennel fronds and serve hot.