To the canister of a blender or food processor, add all ingredients and blend on high speed until smooth, about 1 minute. Divide mixture between two glasses and optionally garnish with fruit. Serve immediately. Extra slushie mixture can be kept in the freezer for up to 1 month by placing freezer-safe glasses inside a large ziplock bag to avoid freezer smells; allow to thaw for about 45 minutes before stirring and serving.