1. Heat farro according to directions.
2. Combine vinegar and next 4 ingredients (through garlic) in a medium bowl; stir with a whisk.
3. Heat 1 tablespoon olive oil in a medium skillet over medium-high heat. Add onion and bell pepper to pan; sauté 2 minutes.
4. Add cooked farro, onion, and bell pepper to vinaigrette; toss to coat. Fold in feta cheese.