If you decide to use rosemary, stick to 1 or 2 strands. Anything more than that and the rosemary flavor can be overpowering.
If you have a really powerful blender, like a Vitamix, don’t blend on the really high settings-the end result is more soup-like than mash.
If you don’t have a blender, you can use a food processor. Or you can pour everything in a bowl and mix with an electric beater. If using an electric beater, the cauliflower chunks might be harder to get rid of, so steam the cauliflower and potatoes longer to make it easier.