Rub chicken breasts with seasoning salt; cover and refrigerate for 30 minutes.
Preheat the oven to 350 degrees F (175 degrees C).
Cook bacon in a large, deep skillet over medium-high heat, turning occasionally, until crisp. Drain on paper towels; set aside.
Combine mustard, honey, corn syrup, mayonnaise, and dried onion flakes in a medium bowl; refrigerate ½ and set remaining ½ aside.
Heat oil in a large skillet over medium heat. Add chicken; cook until browned, 3 to 5 minutes per side. Transfer breasts to a 9x13-inch baking dish. Top each breast with remaining ½ honey-mustard sauce; layer mushrooms and bacon on top. Sprinkle with shredded cheese.
Bake in the preheated oven until cheese is melted and chicken juices run clear, about 15 minutes. Garnish with parsley; serve with reserved ½ honey-mustard sauce.
