Season beef shanks with salt, pepper, and five spice on both sides.
Dredge seasoned shanks lightly with cornstarch, patting off excess.
Heat peanut oil in a pan and sear beef shanks to a dark brown crust on both sides. Transfer the seared beef shanks into the cooking vessel of your slow cooker.
After searing all the shanks, in the same pan, saute ginger, onions, carrots, star anise, and black peppercorns for a minute. Add in the tomato paste and continue roasting for another minute.
Deglaze the pan with chinese cooking wine, scraping all the pan drippings.
Tip the contents of the pan into the slow cooker, on top of the beef shanks. Add in soy sauce, brown sugar, and leeks. Add water until the shanks are barely covered.
Leave to cook on low for 8-10 hours or until the meat is fork tender.