How to Make It
Put the sugar, crushed red pepper and 2 tablespoons of salt in a large, resealable plastic bag and shake to combine. Add the lamb shanks. Seal the bag and shake to evenly coat the shanks. Refrigerate for 4 hours or overnight.
Preheat the oven to 325°. Rinse the lamb shanks and pat dry. In a medium, enameled cast-iron casserole, heat 1 tablespoon of the olive oil. Add the lamb shanks and cook over moderately high heat, turning occasionally, until browned all over, about 10 minutes. Add the onion, carrot, celery, broth, wine, vinegar and Sriracha and bring to a boil. Cover the casserole and braise the shanks in the oven for about 2 hours and 15 minutes, until the meat is very tender.
Remove the lamb from the pot and let cool slightly. Pull the meat from the bones and coarsely shred it. Strain the liquid into a heatproof bowl and spoon off the fat from the surface. Return the liquid to the casserole and boil until reduced to 1 cup, about 15 minutes.
In a very large skillet, heat 2 tablespoons of the oil. Add the eggplant, season with salt and cook over high heat, stirring occasionally, until tender and browned, about 8 minutes. Add the lamb, reduced cooking liquid and sour cream. Stir in the herbs. Cover and keep warm.
In a small skillet, heat the remaining 1 tablespoon of oil. Add the panko and cook over moderate heat, stirring, until golden.
In a large pot of salted boiling water, cook the pasta until nearly al dente. Drain the pasta and add it to the ragù. Cook over moderate heat, tossing until coated with the sauce. Serve the pasta in shallow bowls, sprinkled with the toasted panko.