Trim the rutabagas, cut in half and boil in salted water until tender, about 25 minutes.
Drain, peel and put into bowl of food processor fitted with steel knife.
Add the butter and pulse until melted and incorporated.
Add 1/2 c. of the cream and puree until smooth, adding more cream as necessary, until you have a smooth puree. Open the processor, drop in the sage, and pulse a few times until evenly distributed.
Taste the Swede. If necessary, adjust seasonings with salt and white pepper. Serve immediately or cover with tinfoil and let sit until ready to eat. The Swede can be reheated in a pan or served closer to room temperature, it's nice either way.