Rinse the farro thoroughly under cold water until the water runs clear.
In a medium saucepan, combine the rinsed farro, water or broth, and salt. Bring to a boil, then reduce heat to a simmer. Cover and cook until tender but still chewy, about 25-30 minutes.
Drain any excess liquid and spread the cooked farro on a baking sheet to cool to room temperature.
While the farro cooks, prepare the dressing by whisking together the lemon juice, olive oil, Dijon mustard, garlic, honey, salt, and pepper until emulsified.
Once the farro has cooled, transfer it to a large mixing bowl and toss with half of the dressing while still warm.
Add the shredded chicken, mixed greens, cucumber, red bell pepper, radishes, and herbs to the dressed farro.
Drizzle the remaining dressing over the salad and toss gently to coat all ingredients evenly.
Top with feta cheese and toasted sunflower seeds if using.
Taste and adjust seasoning with additional salt and pepper as needed.
Let the salad rest for about 10 minutes before serving to allow the flavors to meld together.
Serve chilled or at room temperature.
