Prepare the Chicken: Place the chicken breasts between two sheets of cling film or baking parchment. Using a meat mallet or a rolling pin, gently pound them to an even thickness of about 2cm (3/4 inch). This is key for even cooking.

Season the Chicken: In a small bowl, mix together the smoked paprika, garlic powder, oregano, 1 teaspoon of salt, and 1/2 teaspoon of black pepper. Pat the chicken breasts dry with a paper towel and rub the seasoning blend evenly over all sides.

Sauté the Vegetables: Heat the butter in a large, oven-safe frying pan over a medium heat. Add the sliced onions and a pinch of salt. Cook, stirring occasionally, for 10-12 minutes until they are soft, sweet, and beginning to caramelise. Add the sliced mushrooms and cook for another 5-7 minutes until they have released their liquid and started to brown. Stir in the minced garlic and cook for one more minute until fragrant. Remove the onion and mushroom mixture from the pan and set aside.

Sear the Chicken: Add the olive oil to the same pan and increase the heat to medium-high. Carefully place the seasoned chicken breasts in the hot pan. I find that it's best not to overcrowd the pan, so cook in two batches if necessary. Sear the chicken for 4-5 minutes on each side until golden brown and almost cooked through. An internal thermometer should read around 70°C. For food safety, ensure chicken is cooked to a minimum of 74°C; you can check the FSA guidelines for more details.

Smother and Melt: Reduce the heat to low. Arrange the cooked chicken breasts back in the pan. Top each breast generously with the sautéed onion and mushroom mixture. In a separate bowl, toss the grated Monterey Jack and cheddar cheeses together, then sprinkle the mix evenly over the vegetables.

Grill to Perfection: Place the entire pan under a preheated medium-hot grill for 2-4 minutes, or until the cheese is completely melted, bubbling, and just starting to get golden spots. Keep a close eye on it, as it can go from perfect to burnt very quickly!

Rest and Serve: Carefully remove the pan from under the grill. Let the chicken rest in the pan for 5 minutes. This allows the juices to redistribute, ensuring a tender bite. Garnish with fresh chopped parsley and serve immediately.