In a food processor (or a blender) add the cranberries and blend (or process) until they are finely chopped and there are no large pieces.

Move the chopped cranberries to a bowl and add the sugar. Stir to combine. Cover the bowl and move to the fridge to chill for at least 2 hours or overnight.

Drain off any excess juices from the cranberry sugar mixture. You can reserve these juices in a separate container, if you desire.

Add in the drained crushed pineapple and marshmallows. Set aside.

In a stand mixer (or with a hand mixer) whip the heavy cream, adding in the powdered sugar and vanilla to sweeten the whipped cream. Whip until thickened, and soft peaks form. Fold the sweetened whipped cream into the cranberry mixture until combined.

Refrigerate or freeze. When ready to serve, top with pomegranate arils.