In a braiser or large, deep frying pan, warm oil over medium-high heat. When hot but not smoking, add shanks and brown on all sides, 10-12 min. Transfer to a plate. Reduce heat to medium-low, add garlic and saute for 30-40 seconds. Add leeks and saute until translucent, 6-8 min. Return shanks to the pan, add rosemary, salt, pepper, and wine. Raise heat to medium-high and bring to a simmer. Reduce heat to low, cover, and simmer until the shanks are very tender when pierced with a knife, 2 to 2 1/2 hours. Turn once or twice during cooking and add water as necessary to maintain original level of liquid. Serve shanks immediately once tender. To serve, season to taste with salt and pepper, and garnish with rosemary sprigs.