Preheat oven to 425 degrees F.
Set a large, ovenproof skillet over medium heat and add the bacon, onion and garlic. Cook until the bacon is crispy and the onion is translucent, about 8 minutes. Add the mushrooms and cook until soft, about 5 minutes.
Break 1 ½ cup croutons into smaller pieces and stir them into the mushroom mixture. Add 1 ½ cups broth, fold it in and remove from stove. Season generously with pepper.
Crush the remaining 1 ½ cups croutons into crumbs in the food processor or blender, or by putting them into a zipper-seal bag and crushing them with a rolling pin.
Cut a wide slit crosswise in the side of each chicken breast to make a big pocket, being careful not to slice the breasts in half. Divide the crouton mixture among the chicken breasts, pushing it into the pockets. Brush the chicken breasts with mustard.
Set the chicken breasts in the (unwashed) skillet and sprinkle them with the crumbs. Roast in the oven for 16-18 minutes (depending on thickness), until they are cooked through.
Set each of the chicken breasts on a plate. Add the remaining ½ cup chicken broth and the butter to the skillet. Warm over medium heat, stirring. Spoon a little of the pan mixture around each chicken breast, sprinkle with chopped parsley, if using, and serve.