01 - Preheat the oven to 400°F.
02 - Arrange the cherry tomatoes and red onion on a baking sheet. Drizzle with 1 tablespoon olive oil, season with salt and pepper, and toss to coat. Roast for 20–25 minutes, until tomatoes are soft and lightly caramelized.
03 - Bring water or vegetable broth to a boil in a medium saucepan. Add farro, reduce heat, and simmer uncovered for 20–25 minutes, or until tender. Drain excess liquid.
04 - In a small bowl, whisk together remaining olive oil, balsamic vinegar, Dijon mustard, minced garlic, salt, and pepper.
05 - In a large bowl, combine the warm farro, roasted tomatoes and onions, and arugula. Pour over the dressing and toss gently to combine.
06 - Fold in fresh basil leaves and crumbled feta cheese, if using. Serve warm or at room temperature.