In a large saucepan over medium, toast the farro, stirring, until lightly browned and fragrant, about 3 minutes. Add 1 quart water and 1 teaspoon salt; stir. Bring to a simmer over medium-high, then reduce to medium and simmer, uncovered and stirring once or twice, until the farro is tender with a little chew, 25 to 30 minutes.
Meanwhile, in a large bowl, whisk together the vinegar, mustard, oil and ½ teaspoon each salt and pepper. Stir in the onion; set aside. When the farro is done, drain in a colander; cool for 5 to 10 minutes.
Add the still-warm farro to the dressing mixture and toss. Let stand until cooled to room temperature, tossing once or twice. Add the apple, celery and parsley; toss to combine. Taste and season with salt and pepper.
