Cut chicken into cubes and season with salt & pepper. Heat oil in skillet over med heat, add chicken and cook until no longer pink inside, about 10 minutes. Set aside to cool slightly. If desired, you can chop the chicken into smaller pieces, place in food processor to shred, or leave as is. For dressing: mix mayo, mustard, & lemon juice in small bowl. Season with salt & pepper. In large bowl, mix chicken with scallions, cranberries, nuts (optional), grapes, celery & dressing. Serve on sandwich rolls.