Crispy roasted root vegetables like parsnips and celery root add an earthy, caramelized depth to this simple grain salad made with tender, chewy farro. Resist the urge to stir the vegetables during cooking to help the them develop the most color.
Preheat oven to 450°F. Bring 3 cups water and 2 teaspoons salt to a boil in a large saucepan over medium-high. Stir in farro. Cover and reduce heat to low. Cook until farro is tender and water is mostly absorbed, 25 to 30 minutes. Remove from heat, and let stand, covered, 10 minutes.
While farro cooks, toss together carrots, parsnips, onion, celery root, 1/4 cup oil, and 2 teaspoons salt on a rimmed baking sheet; spread in an even layer. Roast in preheated oven until vegetables are tender and lightly browned, 25 to 30 minutes.
Drain any excess water from farro. Stir together farro, chopped parsley, vinegar, pepper, roasted vegetables and any residual oil from roasting pan, and remaining 1/2 teaspoon salt in a large bowl. Let stand at room temperature at least 15 minutes or up to 3 hours, or cover and refrigerate up to 3 days. If chilled, let stand at room temperature 30 minutes before serving.
Just before serving, stir in remaining 2 tablespoons oil. Season to taste with salt, pepper, and vinegar. Garnish with parsley leaves.
Farro and vegetables may be stored in an airtight container in refrigerator up to 3 days.
Buy pearled farro for this recipe; it cooks more quickly than whole-grain.