Preheat oven to 350 degrees F.
Dry out 4 slices of Butter Bread. (If not already stale, toast lightly in oven.) Place in food processor along with salt, pepper, and Italian seasoning and pulse into fine breadcrumbs. Bake breadcrumbs until completely dry and toasty, about 3-5 mins at 350 degrees F.
Increase oven temperature to 375 degrees F and allow to preheat.
Meanwhile, melt butter in a skillet over medium heat; add onions and mushrooms, season with salt and pepper, and cook, stirring, until the mushrooms are dark and softened, about 5 minutes. Remove skillet from heat and allow to cool for 1-2 minutes; then stir in the toasted bread crumbs and set aside.
On a work surface, make 6-7 slits in the top of each chicken breast, about ¾ of the way down, being careful not to cut all the way through. Alternate stuffing each slit with the following: ½ piece ham, ½ piece bacon and a spoonful of mushroom stuffing mixture.
Place stuffed chicken breasts onto a 9×13-inch baking dish. Bake in the 375 degree preheated oven until the chicken is no longer pink and the stuffing is heated through, about 35 minutes. An instant-read thermometer inserted into the center of the chicken should read at least 165 degrees F.
Remove chicken from oven and top with shredded Havarti cheese; broil chicken until the bacon is crisp and cheese is melted and bubbling, about 3-5 minutes.
Recipe adapted from: www.allrecipes.com.