Pat the lamb shanks dry and season them all over with salt and pepper. Heat the olive oil in a large Dutch oven over medium-high heat, then sear the shanks on all sides until browned, about 8 minutes total. Remove the shanks to a plate.

Lower the heat to medium and add the chopped onion to the same pot. Cook until the onion softens and turns golden, about 6 minutes. Stir in the minced garlic and cook for 1 minute more.

Add the ground walnuts to the pot and stir for 2 minutes until they smell toasted. Pour in the pomegranate molasses, beef broth, turmeric, and cinnamon. Bring the mixture to a gentle simmer while scraping up any browned bits from the bottom.

Return the seared lamb shanks to the pot, making sure they are mostly submerged. Cover and simmer on low heat for 2 1/2 to 3 hours, turning the shanks once or twice, until the meat is very tender and pulls easily from the bone.

Taste the sauce and adjust salt or pepper if needed. Spoon the cooked basmati rice into bowls, place one shank on each portion, and ladle the sauce over everything. Finish with pomegranate seeds and chopped parsley.