Preheat the oven to 150°C.
Season the lamb shanks with salt and black pepper.
Brown the lamb shanks in hot oil within a large casserole dish over medium heat.
Add the red wine and simmer until the liquid reduces by one third.
Stir in the quartered onion, garlic head, rosemary sprigs, and anchovies.
Pour in beef stock until the lamb is nearly covered and return to a simmer.
Cook uncovered in the oven for three hours, turning the lamb every 45 minutes.
Sauté the chopped onion, garlic, and rosemary in half the butter for three minutes.
Add the tomatoes and cook until they soften and break down.
Stir in the drained flageolet beans and heat through with seasoning.
Melt in the remaining butter and stir in the fresh parsley.
Remove the cooked lamb shanks and keep warm on a plate.
Boil the remaining cooking liquid on the hob until slightly reduced.
Strain the liquid into a clean pan, season, and whisk in butter until glossy.
Serve the lamb shanks over the beans and drizzle with the reduced sauce.