To make the lamb ragù, season the lamb shanks with salt and pepper. Heat a large pressure cooker over high heat. Add 2 teaspoons of the oil and then the lamb and cook, turning occasionally for about 8 minutes, or until browned on all sides. Remove the lamb from the pot and discard the fat.
Reduce the heat to medium. Add the remaining 2 teaspoons oil, the sliced onion, garlic and chilli to the pot and cook, stirring often, for about 8 minutes, or until the onion is tender and beginning to caramelise. Add 1 ½ cups of the crushed tomatoes and the water and return the lamb to the pot.
Lock the pressure cooker lid in place and bring to high pressure over high heat, about 15 minutes. Reduce the heat to low to stabilise the pressure and cook the lamb for 50 minutes. Remove from the heat and allow the pressure to subside on its own, about 20 minutes.
Unlock the pressure cooker and remove the lid, tilting it away from you to allow the steam to escape. Spoon any scum and oil from the top of the braising liquid. Set aside until the lamb is cool enough to handle, then pull the meat from the bones; discard the bones and cartilage and set the meat aside.
Add the remaining ½ cup crushed tomatoes to the pressure cooker, bring to a boil over medium-high heat, and boil for about 5 minutes, or until the sauce is reduced by one-third. Add the lamb meat to the sauce and stir in the olives. Remove the pot from the heat and season the ragù to taste with salt and pepper. Cover to keep hot while the pasta cooks.
Meanwhile, to make the gremolata, in a small bowl, mix the parsley, lemon zest, and chopped chilli. Set aside.
To cook the pasta, bring a large pot of salted water to a boil over high heat. Add the pasta and cook, stirring occasionally, for about 8 minutes, or until tender but ﬁrm to the bite. Drain the pasta, reserving about ½ cup of the cooking water.
Return the ragù to a gentle simmer over medium-low heat. Gently stir the pasta into the ragù and cook for about 1 minute, adding the reserved pasta water to moisten the mixture as needed. Remove the pot from the heat and stir in the butter and Parmesan cheese.
Divide the pasta among four bowls. Sprinkle the gremolata and a little more Parmesan cheese over the top and serve immediately.