01 - Preheat oven to 400°F. Line a baking sheet with parchment paper. Toss plum quarters with 1 tbsp olive oil and 1/2 tsp honey. Spread on prepared baking sheet and roast for 15–18 minutes, until soft and slightly caramelized. Cool to room temperature.
02 - Rinse farro under cold water. In a medium saucepan, bring 2 cups water and 1/2 tsp salt to a boil. Add farro, reduce heat, and simmer uncovered for 25–30 minutes, or until tender but chewy. Drain and let cool slightly.
03 - In a small bowl, whisk together 3 tbsp olive oil, balsamic vinegar, Dijon mustard, 1 tsp honey, salt, and pepper until emulsified.
04 - In a large bowl, combine cooked farro, arugula, red onion, toasted walnuts, and cheese (if using). Drizzle with dressing and toss gently to coat evenly.
05 - Top salad with roasted plums. Serve immediately or at room temperature.