Mashed Sweet Potatoes with Pecans & Marshmallows

lbs. sweet potatoes or yams, rinsed and scrubbed 2 Tbsp. olive oil 2 Tbsp. kosher salt 6 oz. heavy cream 4 oz. butter ¼ tsp. nutmeg 2 Tbsp. pure maple syrup 5 oz. mini-marshmallows ¼ cup pecans