Beetroots freeze well.
I roast them whole for 45 mins at 180c. Remove the skins once cooled and freeze in containers. Have them whole or crinkle cut, both types freeze well.
Also pickle a load in Sarsons classic malt pickling vinegar. That pickling vinegar keeps any vinegar taste very mild, which is how I like it. Same method as above but instated of freezing, put beetroot into a kilnar jar and add the sarsons vinegar.