Blend all ingredients (except garnish) in a blender until smooth.
Pour mixture into 2 large casserole dishes. You want the depth of the mixture to be no more than about half an inch. Split into as many containers as needed. Two large casserole dishes should work though.
Freeze for 1 hour.
Remove from the freezer and scrape/mix with a fork, incorporating the frozen edges into the middle of the dish.
Freeze for 30 minutes.
Scrape/mix with a fork.
Freeze for 30 minutes.
Scrape/mix with a fork.
Freeze for 30 minutes.
Scrape/mix with a fork.
Freeze for 30 minutes.
Scrape/mix with a fork.
Freeze for 30 minutes.
You guessed it, scrape/mix with a fork! At this point it should be done. If not, continue to freeze and scrape in 30 minute increments until the mixture is completely frozen into small crystals.