1. In a saucepan cook whole beets.
2. Cut and cook again.
3. Drain out from water.
4. Peel the skin and dice.
5. In a bowl beat butter, mustard, and tarragon vinegar.
6. Transfer beets to the pan and cook with 1 tablespoon water.
7. Transfer into a serving bowl.
8. Cut in mustard butter in small pieces.