Bring a large pot of water to boil. Add in the farro and cook until tender, about 12 minutes. Drain the water from the pot.
In a small bowl or cup, whisk the red wine vinegar into the olive oil, then whisk in the fish sauce to taste (It's an intensely salty and fishy flavor and I find that I often prefer more than other people do!). Dress the farro and toss with the radishes and scallions.