You can toss the farro with the fennel mixture up to one day ahead; add the mint, radicchio, and fennel fronds just before serving.
In a bowl, whisk together lemon juice, honey, shallot, 1/2 teaspoon salt, 1/4 teaspoon pepper, and oil. Add raisins and sliced fennel; toss to coat.
In a medium saucepan, cover farro with 4 inches water and bring to a boil; season with salt. Reduce heat and simmer until tender, 12 to 15 minutes. Drain and let cool completely.
Add farro to bowl with mint, radicchio, and fennel fronds. Season with additional salt, if desired. Serve.