Bring a medium pot of salted water to a boil and add the farro. Simmer until tender but still chewy, about 20–25 minutes for pearled farro. Drain and return to the pot off the heat.
While the farro cooks, combine 1 tablespoon olive oil, minced garlic, 1 tablespoon lemon juice, 1/2 teaspoon salt, 1/4 teaspoon pepper and the dried oregano in a shallow dish. Add chicken breasts and turn to coat; let sit at room temperature for 10–15 minutes.
Heat a large skillet over medium-high heat. Add the remaining 1 tablespoon olive oil and sear the chicken 5–7 minutes per side until golden and an instant-read thermometer reads 160°F. Transfer to a cutting board and let rest 8 minutes (carryover will bring it to ~165°F).
While the chicken rests, toast the pine nuts in a small dry skillet over medium, stirring, until fragrant and lightly browned, about 2 minutes. Set aside.
Whisk together the remaining olive oil, 2 tablespoons lemon juice, the sumac, remaining 1/2 teaspoon salt and 1/4 teaspoon pepper to make the dressing.
Toss the hot drained farro with half the dressing so the grain absorbs flavor and cools slightly. Fold in chopped parsley to distribute freshness through the grain.
Cut rested chicken into slices. In a large bowl, combine the dressed farro, cucumbers, tomatoes, red onion, and olives. Add the chicken slices on top and drizzle remaining dressing over the assembled bowl.
Scatter crumbled feta and toasted pine nuts over each serving. Taste and adjust seasoning with extra lemon juice, salt, or pepper if needed, then serve warm or at room temperature.