In the bowl of your slow cooker, combine the potatoes, parmesan rind (if using),heavy cream, milk, garlic, and sage. Cover and cook on high for 4-5 hours or on low for 6-8 hours, until the potatoes are fork tender. Switch the slow cooker to warm.
Drain the potatoes, reserving all of the cream. Discard the thyme, sage, and parmesan rind (if used). Press the potatoes through a potato ricer, adding them back to the slow cooker. Alternately, you can add the potatoes back to the slow cooker and mash with a potato masher.
Add 1 1/2 cups of the reserved cream and the butter. Continue to add more of the cream until your desired consistency is reached. Season the potatoes with salt and pepper. Stir in the cheese, cover, and cook another 15 minutes, or until melted and creamy. Keep the potatoes covered, on warm, for up to 4 hours.
Serve the potatoes topped with additional fresh herbs and butter.