1. Place 2 1/2 cups water in medium saucepan, and bring to a boil. Add farro, reduce heat to medium-low, and simmer, partially covered, 30 minutes. Stir in half of sunchokes, and simmer, partially covered, 10 minutes more. Remove from heat, stir in remaining sunchokes, cover, and let stand 5 minutes. Transfer to salad bowl to cool completely.
2. Toast sunflower seeds in small dry skillet over medium-low heat 5 minutes, or until lightly browned and nutty, shaking often to prevent scorching. Transfer to plate to cool.
3. Whisk together vinegar and mustard in bowl. Whisk in oil until emulsified, and season with salt and pepper, if desired.
4. Toss together farro mixture, spinach, and green onions. Add dressing, and toss to coat. Season with salt and pepper, if desired, and top with sunflower seeds.