01 - Preheat the oven to 300°F. Line a baking sheet with parchment paper. Arrange halved tomatoes cut side up on the prepared sheet. Drizzle with olive oil, sprinkle with salt, pepper, oregano, and scatter garlic slices over. Roast for 1½ hours, until tomatoes are deeply caramelized and shriveled but still moist. Let cool.
02 - While tomatoes roast, rinse farro under cold water. Combine farro, water, and salt in a saucepan. Bring to a boil, then reduce heat and simmer for 25–30 minutes, until farro is tender but chewy. Drain any excess water and let farro cool to room temperature.
03 - In a large bowl, whisk together olive oil, red wine vinegar, Dijon mustard, honey, salt, and pepper for the dressing.
04 - Add cooled farro, slow-roasted tomatoes (with their juices), red onion, basil, parsley, and arugula to the bowl. Toss gently to combine and coat everything in the dressing.
05 - Top with crumbled feta. Toss lightly and serve at room temperature or slightly chilled.