To avoid long cooking times and to insure the rutabaga is soft enough to mash, I use a pressure cooker to cook them so the directions are for that appliance

Place rutabaga chunks in the pressure cooker

Add salt if using and the water.

Seal pot and process for 15 minutes at 10 psi or "vegetable" setting on electronic models

Let the pressure release naturally, i.e. don't open the vent.

When pressure has dropped, open pot and drain the rutabagas

Add the butter and pepper and mash with a potato masher or electric mixer.