Remove outer leaves, wash and chop cauliflower into large pieces. Discard the cores.
Add to a large slow cooker, along with garlic and enough water to cover. Stir to make sure no cauliflower is sticking out, cover and cook on Low for 8 hours or on High for 4 hours.
Drain cauliflower and garlic (do not discard) in a colander really well, otherwise mashed cauliflower consistency will be off. Return to a slow cooker, add butter and salt, and stir to let butter melt faster.
Puree with an immersion blender until smooth. Be patient, it will take 4-5 minutes because cauliflower has no starch unlike potatoes.
Add chives, stir and serve like you would serve mashed potatoes.
Video
Notes
Store: Refrigerate covered for up to 1 day. Cauliflower mashed potatoes taste best consumed within 24-36 hours.
Add more salt to taste if using unsalted butter (I used salted).