Lightly season the chicken breasts with salt and pepper on both sides. In a large skillet over medium-low heat, add 2 tablespoons of the butter, the onions and the garlic. Cook until the onions are nice and caramelized, about 30 minutes. Remove the onions from the pan and set aside.

Increase the heat to medium-high, add the remaining 2 tablespoons of butter and pan-sear the chicken on both sides until they are a deep golden brown. Remove from pan and set aside. (Chicken will not be fully cooking during this stage)

De-glaze the pan with stock. Using a rubber spatula, scrape up and mix in any remaining bits of chicken stuck to the bottom of the pan. Let simmer for 3 to 5 minutes.

Reduce the heat to medium-low to low, add the cooking sherry, garlic, Worcestershire, thyme, salt and pepper. Let simmer for 6 minutes, or until the sauce has reduced by half and thickened slightly.

Add caramelized onions back to the pan and nestle in the chicken, and simmer until the chicken is cooked all the way through, about 10 minutes or until it has reached an internal temperature of 165°F.

Spoon the onions and some sauce over the chicken and then top with a combination of gruyere and mozzarella cheese. Bake in the oven for 7 to 8 mins at 275°F or until the cheese is melted and bubbly. Spoon a little bit of onion gravy on top and garnish with fresh thyme.