Coat slow cooker with vegetable oil spray. Melt 2 tablespoons butter in 12_inch skillet over medium heat. Add onion and cook until softened, about 5 minutes. Add garlic and cook until fragrant, about 30 seconds. Stir in rice and cook, stirring often, until ends of rice kernels are transparent, about 3 minutes. Stir in wine and cook until fully absorbed, about 1 minute; transfer to slow cooker.
Bring broth and water to boil in saucepan. Stir 3 cups hot broth mixture into slow cooker. Cover and cook until rice is tender, about 2 hours on high.
Return reserved broth mixture to boil. Slowly stream 2 cups more hot broth mixture into slow cooker, stirring gently, until liquid is absorbed and risotto is creamy, about 1 minute.
Gently stir in Parmesan, chives, remaining 2 tablespoons butter, and lemon juice. (Adjust risotto consistency with additional hot broth mixture as needed.) Season with salt and pepper to taste and serve. (This dish does not hold well on warm setting.).