Preheat oven to 425°F; line a rimmed baking pan with nonstick foil. Prepare farro as label directs; cool.
Toss onion, potato, 1 tablespoon oil, ½ teaspoon salt and cinnamon in a large bowl; spread on prepared pan. Roast onion mixture 25 minutes or until golden brown and tender, stirring once.
Whisk lemon juice, parsley, honey and remaining 1 teaspoon salt in a separate large bowl; whisking constantly, slowly drizzle in remaining 3 tablespoons oil to emulsify. Fold in arugula, Mandarins, farro and onion mixture. Makes about 7 cups.
Serve salad sprinkled with pomegranate arils and pepitas.