Season the meat and dust with flour. Heat the clarified butter in a pan. Brown the shanks for approx. 3 mins. on each side. Remove, reduce the heat. Wipe away the cooking fat. Add the onions and garlic, sauté briefly. Add the bay leaf, pour in the wine and stock, bring to the boil, reduce the heat, return the meat to the pan, cover and braise for approx. 1½ hrs. Remove the shanks from the sauce, pull apart using two forks. Add the spinach, lemon zest, lemon juice and single cream to the sauce along with the meat. Simmer for approx. 15 mins., season.
Cook the rigatoni in salted water until just al dente, drain. Add the rigatoni to the ragù, heat through, plate up, sprinkle with parmesan.