In a blender, combine the oil, vinegar, mustard, and onion. Cover and gradually increase the speed to high; blend 2-3 minutes or until smooth. Stir in the poppy seeds and season to taste with salt and pepper. Chill until ready to use.
In a small sautee pan, heat 1/2-inch of grapeseed oil. When hot, add the sliced onions. Fry for 2-3 minutes, or until light golden brown. Transfer to a plate lined with a paper towel.
Meanwhile, add the farro and water to a saucepan. Bring to a boil, then reduce heat. Cover and simmer for 20-30 minutes or until the farro is soft and the water is absorbed. Transfer the cooked farro to a large mixing bowl.
Stir in the arugula, peaches, cucumber, fried onions, and half of the dressing. Chill thoroughly and serve with additional dressing, if desired.