Preheat your oven to 350F/180C. Season the chicken with salt and pepper and arrange it in a baking dish. Set aside.
Step 2
Add the butter to a nonstick pan and place it over medium heat. Thinly slice the onion and transfer it to the pan. Add a pinch of salt and stir for 2 minutes until coated with the butter.
Step 3
Drop the heat to medium-low and cook for 15-20 minutes, stirring regularly, until soft and caramelized. Taste and season if needed. Set aside.
Step 4
Transfer the chicken to the oven and bake the chicken for 15-20 minutes until cooked through. Top with the caramelized onions and shredded cheese. Return to the oven and broil for 2-3 minutes until the cheese has melted.
Step 5
Take the baking dish out of the oven and let the chicken rest for 5 minutes. Chop the parsley and sprinkle it on top. Serve immediately.