If you can't find precooked farro, substitute precooked or boil-in-bag brown rice.
Heat farro according to directions.
Combine vinegar and next 4 ingredients (through garlic) in a medium bowl; stir with a whisk.
Heat 1 tablespoon olive oil in a medium skillet over medium-high heat. Add onion and bell pepper to pan; sauté 2 minutes.
Add cooked farro, onion, and bell pepper to vinaigrette; toss to coat. Fold in feta cheese.
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