Paste the garlic with the salt; add to a bowl with the vinegar and let sit while preparing the other ingredients. (Pasting the garlic and allowing it to sit in the vinegar will mellow the flavor.)
Add the pepper, mustard, and oregano, and whisk to mix together. Slowly whisk in the olive oil.
Rinse the farro. Bring two cups of water and a pinch of salt to boil; add farro. When it returns to boil, reduce the heat to medium-low, cover, and let simmer for 30 minutes.
Remove from heat and let stand, covered, for about 10 minutes.
While the farro cooks, get the vegetables ready. Slice the tomatoes in half (in quarters if large) and dice the cucumber. Thinly slice the scallions and measure out just the whites and light greens. You can reserve the dark greens for garnish. Rinse and drain the chickpeas and slice the olives.
Once the farro has cooled slightly, add the vegetables, feta cheese, and ¼ cup of the dressing. Mix well. You can serve it immediately or chilled.