1. Bring a large pot of water to boil. Add cauliflower florets. Cook until fork tender but not mushy, approximately 10-15 minutes.
2. Remove cauliflower and place pieces in a large bowl. Use a potato masher to break up the largest chunks. Pour into a blender or large food processor, but keep the bowl to the side.
3. Add milk, rosemary and salt to cauliflower and blend to desired consistency. Return to the original bowl and stir in cheddar cheese until melted through.