Unthaw IS NOT A WORD.
Think about it. If thaw is to melt ice from the meat, to unthaw would mean to refreeze the meat. Every decent cook knows that once meat is thawed it is to be cooked, not refrozen uncooked — the certain path to freezer burn. Therefore to thaw then unthaw (not a real word) is to refreeze not to thaw (the only word).