Preheat oven to 425 degrees F. Line a 15x10x1-inch baking pan with foil; set aside. In a large bowl combine Brussels sprouts, apples, onion, pancetta, oil, salt, mustard seeds, and pepper. Transfer mixture to the prepared baking pan. Roast, uncovered, for 25 to 30 minutes or until sprouts are tender, stirring once.
In a small bowl whisk together vinegar, honey, and Dijon mustard. Drizzle vinegar mixture over sprout mixture; toss gently to coat.