Preheat your oven to 375°F (190°C) and prepare a 9x13-inch baking dish by lightly greasing it with butter or cooking spray.

Heat 2 tablespoons of olive oil in a large skillet over medium heat, add the thinly sliced onions with a pinch of salt, and cook for 25-30 minutes, stirring occasionally, until deeply caramelized and golden brown.

While onions caramelize, season chicken breasts with salt and pepper, then cook in a separate pan over medium-high heat for 6-7 minutes per side until fully cooked through and internal temperature reaches 165°F.

Remove cooked chicken from heat, let rest for 5 minutes, then shred or dice into bite-sized pieces and set aside.

Bring a large pot of salted water to boil, add elbow macaroni, and cook for 2 minutes less than package directions to achieve al dente texture, then drain and set aside.

In a large saucepan, melt 4 tablespoons of butter over medium heat, add minced garlic and cook for 30 seconds until fragrant, then whisk in flour to create a roux and cook for 2 minutes while stirring constantly.

Gradually pour in chicken broth while whisking continuously to prevent lumps, then add milk and heavy cream, whisking until smooth and combined.

Bring the sauce mixture to a gentle simmer, stirring frequently, and cook for 5-7 minutes until thickened enough to coat the back of a spoon.

Remove sauce from heat and stir in Gruyere cheese, mozzarella cheese, and Parmesan cheese, whisking until completely melted and smooth, then add Worcestershire sauce, Dijon mustard, salt, pepper, and fresh thyme.

Add the cooked macaroni, shredded chicken, and caramelized onions to the cheese sauce, folding everything together gently but thoroughly to ensure even distribution.

Transfer the macaroni mixture to your prepared baking dish, spreading it evenly and pressing down gently to eliminate air pockets.

In a small bowl, combine panko breadcrumbs with 2 tablespoons of melted butter, mixing until breadcrumbs are evenly coated, then sprinkle this mixture evenly over the top of the casserole.

Bake uncovered for 25-30 minutes until the top is golden brown and crispy, the edges are bubbling, and the casserole is heated through completely.

Remove from oven and let rest for 5-10 minutes before serving to allow the sauce to set slightly and make serving easier.