Tomas
According to my Gem guide to Freezing you can
it says can be frozen whole if firm and small or larger ones can be sliced Wash well, cook in boiling water until tender, 5-10 minutes for the smallest up to 50 minutes for the big ones.
Drain and allow to cool, then rub the skins off. Pack small beets into polythene bags and slice large ones into rigid containers.
freezer life, 6 months
Thawing leave to thaw in unopened containers in the fridge for 6- 8 hours or at room temp for 2-4 hours.
best wishes
Sue