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 1/2 and set remaining 1/2 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 1/2 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 1/2 honey-mustard sauce.