In the insert of a 5–7-quart slow cooker, add the potatoes, milk, herb bundle and garlic, if using. Season with salt and pepper. Cover and cook on low for 6 to 8 hours, or until the potatoes are fork tender. Stir occasionally, if possible.
Discard the herb bundle, if using. Mash the potatoes in the slow cooker insert until smooth. Fold in the cheese and butter, if using. Add more milk or cream, if needed, until you’ve reached your desired consistency. Season with salt and pepper.
Serve the mashed potatoes right from the slow cooker or scrape into a serving bowl. Top top with a knob of butter and garnish with crispy sage leaves, if desired. Sprinkle with flaky sea salt and more black pepper.