Preheat oven to 400°F (205°C).
Season shanks with salt and pepper. Heat oil in a heavy-bottomed pan and brown shanks until meat caramelizes. Add onion and garlic and continue to brown. Transfer meat and onion to a large roasting pan.
Deglaze pan with wine, scraping up brown bits from bottom of pan. Pour wine over shanks and onions and add enough stock to cover shanks. Add 3 of the sprigs of thyme, bay leaves and peppercorns. Bring to a boil on stove-top, cover, and put in oven for 1 1/2 to 2 hours. Remove shanks; strain broth and cool. Remove meat from bones after it has cooled.
Skim off fat from broth. Add rosemary and remaining thyme sprigs; reduce broth by half. Combine sweet potatoes, peppers, garlic, onion and olive oil; oven-roast covered for 30 minutes.
When ready to serve, melt butter and lightly brush browned lamb. Add vegetables and broth. Simmer until everything is heated thoroughly, about 10 min. Serve hot with parsley.