Place the farro in a bowl and cover with cold water. Swirl it around a few times, then drain well. Add the farro and 2½ cups water or broth to a saucepan and bring to a boil. Reduce to a simmer and cook uncovered for 25–30 minutes, or until tender with a bit of chew. Drain and let cool for a few minutes.
While the farro cooks, prep the vegetables. Halve or quarter the tomatoes depending on their size, peel and dice the cucumber, finely dice the red bell pepper and red onion, slice the Kalamata olives, and chop the parsley.
In a small bowl or jar, whisk together the olive oil, red wine vinegar, garlic, oregano, Dijon mustard, salt, and pepper.
If you prefer a milder red onion flavor, place the diced onion in a small bowl and toss with a spoonful or two of the dressing. Let it sit and mellow while the farro finishes cooking.
In a large bowl, combine the farro, tomatoes, cucumber, red bell pepper, Kalamata olives, red onion, and parsley. Pour the dressing over the salad and toss well.
Gently fold in the arugula and feta.
Serve at room temperature, or refrigerate until ready to serve. If chilled, give the salad a gentle toss before serving.