BAKING METHOD: Wash the yams and pet dry with paper towel. Fold the yams in heavy duty aluminum foil and bake at 350F for about 2 hours or until tender (baking time varies depending on the size of yams).
BOILING METHOD: Peel and cut the yams into large pieces. Add enough water just to cover the yams and bring to a boil. Simmer the yams until cooked through for about 30 minutes. Remove the water reserving 1/2 cup, which could be used during mashing to regulate the texture.
Once the yams are cooked, set them aside.
In a small skillet melt the butter and add pistachios and rosemary. Cook on medium heat for about 1 minute.
Remove the pistachios and rosemary from the pan and save for later.
Place the yams into a large bowl and add the melted butter and cream, and mash with the potato masher. During this step, taste the yams, add a little maple syrup if not sweet enough or a little of the reserved water to give it a little smoother consistency. Once satisfied with the taste, add feta cheese and mash a little more.
Transfer mashed yams onto a serving platter and garnish with reserved pistachios, rosemary and feta crumbles.