Rinse and dry the lamb shanks, and season them liberally with salt and pepper. In a very large heavy-bottomed Dutch oven, heat the olive oil over medium-high heat until smoking. Add the lamb shanks, 5 at a time, and sear until dark golden brown all over, 10 to 12 minutes per batch. Remove the shanks and set them aside.
Add the onions, garlic, carrots, and leeks to the pot and cook until softened, 8-10 minutes. Use only the white and light green parts of the Leaks only, trimmed, halved lengthwise, cut crosswise into thin half-moons, rinsed thoroughly, and drained.
Add the wine, tomato sauce, and sock to the vegetables and bring to a boil. Return the lamb shanks to the pot and bring back to a boil. Cover the pot tightly, place it in the oven, and bake for about 1.5 hours, until the meat is fork-tender.
Remove the pot from the oven, check the sauce for seasoning, and then add the grapes. Stir them in gently, and serve directly from the pot.