Combine 16 cups water, salted as you would for pasta, and Grano in a pot. Bring to a boil, then reduce heat to medium. Simmer, stirring frequently, until Grano is tender but still chewy, about 30-35 minutes. Drain well.
Whisk oil and vinegar in large bowl. Season with salt. Add Grano; toss. Cool.
Meanwhile, cook fava beans in pot of boiling salted water for 3 minutes. Add peas and cook until just tender, about 1 minute longer. Drain. Cool in bowl of ice water. Drain. Add fava beans and peas to Grano. (Can be made 4 hours ahead. Cover; chill.)
Add basil, arugula and tomatoes to Grano mixture. Toss to combine. Season generously with salt. Spoon salad onto platter. Garnish with additional arugula.