Make the honey mustard sauce: In a small bowl, whisk together Dijon mustard, honey, mayonnaise, lemon juice, and garlic powder. Set aside.
Prep the chicken: Flatten chicken breasts slightly using a meat mallet or rolling pin between two pieces of plastic wrap for even cooking. Season both sides with salt and pepper.
Sear the chicken: Heat olive oil in a skillet or grill pan over medium-high heat. Cook chicken for 4-5 minutes per side until browned and mostly cooked through. Remove from heat.
Cook bacon and mushrooms: In the same pan, cook bacon strips until crispy. Drain on paper towels. Add sliced mushrooms to the pan with butter (or bacon grease) and sauté until softened and golden, about 5-7 minutes.
Assemble: Preheat oven to 375°F. Place chicken breasts in a baking dish. Spoon honey mustard sauce over each piece. Top with sautéed mushrooms, 2 bacon slices per breast, and generous amounts of both cheeses.
Bake: Bake for 10-15 minutes until cheese is melted and bubbly, and internal temperature reaches 165°F.
Serve: Let rest for 5 minutes. Drizzle with remaining honey mustard sauce before serving.