1. Gently pound the chicken breasts to between ½- to ¼-inch thickness. For the honey mustard, in a small bowl, whisk together the next 4 ingredients (mustard through vinegar). Set aside.
2. Melt butter in a large nonstick skillet over medium heat; add mushrooms and cook for 8 minutes, or until browned and tender. Remove from pan and set aside.
3. Coat skillet with cooking spray and turn heat to medium-high. Add chicken and cook for 3 minutes, or until underside is golden brown. Turn, and top each breast with ¼ cup mushrooms. Add pepper to taste, and sprinkle each breast with 3 tablespoons cheese and 2 teaspoons bacon. Add 2 tablespoons water to pan and immediately cover. Cook for 2 minutes, or until cheese is melted and chicken is cooked through.
4. To serve, top each breast with 1 tablespoon green onions and drizzle with a tablespoon of the honey-mustard sauce.