In a medium saucepan, cover the farro with 2 inches of cold water and bring to a boil. Cook over high heat for 5 minutes. Turn off the heat and let the farro stand until plump and al dente, 45 to 60 minutes. Drain the farro in a colander, shaking off the excess water. Let stand until cool; shake the colander occasionally to dry the farro.
In a large bowl, whisk the lemon juice with the olive oil and season generously with salt and pepper. Add the farro, tomatoes, scallions, parsley and mint and toss. Season with salt and pepper, gently fold in the arugula and serve.
Make Ahead: The farro can be prepared through Step 1 and refrigerated for up to 2 days. The finished salad can stand at room temperature for up to 30 minutes before serving.