Place the rutabagas in a large pot and cover with cold water.
Add salt and bring to a rapid simmer.
Cook for about 25 minutes or until the rutabagas are very tender when pierced with a fork.
Remove from stovetop; drain thoroughly and transfer to a large mixing bowl.
Add 1 tbsp. butter, 1/2 tsp. salt, 1/4 tsp. black pepper and 1 tbsp. brown sugar, if using; mash the rutabagas until all the large lumps are gone; alternatively, puree the mixture until smooth with an immersion blender.
Taste and adjust the seasonings, as needed; transfer to a serving bowl.