Pat the lamb neck dry and season it with half the salt and pepper. Heat the olive oil in a Dutch oven over medium-high heat and brown the lamb in batches until all sides are seared. Remove the meat and set it aside.
Lower the heat to medium and add the onion to the same pot. Cook for 5 minutes until softened. Stir in the garlic and cook for 1 minute more.
Return the lamb to the pot. Add the diced tomatoes, olives, beef broth, thyme, bay leaf, and remaining salt and pepper. Bring the mixture to a simmer.
Cover the pot and reduce the heat to low. Cook for 2 hours, stirring once or twice, until the lamb is fork-tender and the broth has thickened slightly.
While the stew simmers, bring the water to a boil in a medium saucepan. Whisk in the polenta and reduce the heat to low. Cook for 20 minutes, stirring often, until thick and creamy.
Stir the butter and Parmesan into the polenta. Taste and adjust salt if needed. Serve the stew ladled over scoops of the hot polenta.