In an 8-qt. saucepan, heat the olive oil over medium-high. Season the lamb shanks with salt and pepper and, working in batches, add to the pot and cook, turning as needed, until golden brown, about 10 minutes. Transfer the shanks to a plate. Add the peppercorns, celery, garlic, rosemary, thyme, bay leaf, and onion to the pot and cook, stirring, until the vegetables begin to brown, about 10 minutes. Stir in the tomato paste and cook until lightly caramelized, 2 to 3 minutes. Pour in the red wine, bring to a boil, and cook until reduced by half, 4 to 5 minutes. Return the shanks to the pot along with 4 cups water and bring to a boil. Reduce the heat to maintain a gentle simmer and cook, covered, until the lamb is very tender, 2-1⁄2 to 3 hours.