1) Place chicken in a zip-lock bag and pour 1 cup of balsamic vinaigrette in bag with chicken. Marinate for at least 2 hours, but best if marinated overnight. Take chicken out of zip-lock bag and discard marinade.
2) In a grill pan, grill chicken breasts, about 5 to 6 minutes on each side until cooked through. Place yellow bell pepper slices in grill pan and grill about 2 minutes on each side.
3) Whisk together the rest of balsamic vinaigrette and mustard in small bowl.
4) Cook pasta according to package directions; drain. Spread pasta in a single layer on baking sheet. Drizzle with 1/2 tbsp olive oil. Let cool 10 minutes.
5) Toss together pasta, chicken, balsamic vinaigrette mixture, spinach, and remaining 3 ingredients in a large bowl. Serve immediately, or, if desired, cover and chill 1 hour before serving.