Boil the farro and a pinch of salt in plenty of water. Cook until al dente, about 20 minutes. Pour into a colander and rinse under cold water until cool. Drain very well and transfer into a serving bowl.
Add the beets and cucumber to the serving bowl.
Prepare the dressing: In a blender or a food processor, combine the onions, garlic, mint, rice vinegar, sea salt and pepper. Turn on the motor and add the olive oil in a slow steady stream.
Drizzle the dressing over the salad and toss to combine. Add feta and toss gently. Taste for seasonings.