01 - Preheat the oven to 400°F (200°C).
02 - Combine farro, water, and salt in a medium saucepan. Bring to a boil over medium-high heat, then lower to a simmer. Cook uncovered for 25 to 30 minutes until the farro is tender yet retains a pleasant chew. Drain off any remaining liquid and transfer the farro to a bowl to cool.
03 - Place plum halves cut side up on a baking sheet. Drizzle with olive oil and honey or maple syrup, scatter thyme leaves on top, and season with black pepper. Roast for 18 to 20 minutes until softened and caramelized. Allow to cool slightly, then slice each half into wedges.
04 - In a small bowl, whisk together extra virgin olive oil, balsamic vinegar, Dijon mustard, honey or maple syrup, salt, and black pepper until well emulsified.
05 - In a large mixing bowl, gently combine the cooled farro, baby arugula or mixed greens, sliced red onion, and toasted walnuts. Pour the dressing over and toss until coated.
06 - Distribute the salad onto plates. Top each with roasted plum wedges and a sprinkle of crumbled feta if using. Serve immediately, or refrigerate and serve chilled.