01 - Set oven temperature to 300°F. Line a rimmed baking sheet with parchment paper for easy cleanup.
02 - Arrange tomato halves cut side up on prepared baking sheet. Drizzle with 3 tablespoons olive oil, then season with 1 teaspoon sea salt, ½ teaspoon black pepper, and 1 teaspoon dried oregano. Scatter garlic slices evenly over top. Roast for 1 hour 30 minutes until tomatoes are deeply caramelized, shriveled, yet still moist. Allow to cool on pan.
03 - Rinse farro thoroughly under cold running water. Transfer to medium saucepan along with 3 cups water and ½ teaspoon salt. Bring to boil over high heat, then reduce to medium-low and simmer covered for 25 to 30 minutes until grains are tender but still pleasantly chewy. Drain any excess liquid and spread farro on large platter or baking sheet to cool to room temperature.
04 - In large mixing bowl, whisk together 3 tablespoons olive oil, 2 tablespoons red wine vinegar, 1 teaspoon Dijon mustard, 1 teaspoon honey, and salt and pepper to taste until fully emulsified.
05 - Add cooled farro, slow-roasted tomatoes with all their flavorful juices, sliced red onion, torn basil, chopped parsley, and arugula to the bowl with dressing. Gently toss until all components are evenly coated.
06 - Sprinkle crumbled feta over top and toss lightly just once. Serve immediately at room temperature, or refrigerate for 1 hour to serve slightly chilled.