Melt butter in skillet over medium heat. Add chopped mushrooms and sauté until tender (about 3 minutes). Set aside.
Mix spinach, cream cheese and chives in bowl. Add mushrooms and season with salt and pepper.
Use a knife to slice a pouch in each chicken breast. Divide spinach mixture between the six breasts, spooning into pouch and then folding the chicken down to cover the spinach.
Place chicken breasts on cookie sheet. Spread a dollop of Dijon mustard on top of each breast.
Bake chicken for about 20 minutes, or until golden brown.
Notes
If preparing this recipe as gluten free, just be sure to use a brand of Dijon mustard that is designated as GF.