01 - Combine farro, water, and salt in a medium saucepan. Bring to a boil over high heat, then reduce to low heat, cover, and simmer for 25 to 30 minutes until farro is tender and water is absorbed. Drain any excess liquid and let cool completely.
02 - Whisk together extra-virgin olive oil, lemon juice, orange juice, Dijon mustard, honey, salt, and black pepper in a large bowl until emulsified and smooth.
03 - Add cooled farro, sliced fennel, orange segments, grapefruit segments, shallot, and baby arugula to the bowl with the dressing. Toss gently but thoroughly to coat all ingredients evenly.
04 - Transfer salad to a serving platter or shallow bowl. Sprinkle toasted pistachios and reserved fennel fronds over the top as finishing touches.
05 - Serve immediately for optimal texture, or refrigerate for up to 2 hours to allow flavors to meld before serving.