Add half of the butter and oil to a large oven-safe pan over medium heat then add the sliced onion. Fry for 15mins, stirring somewhat frequently, until the onions turn soft and golden.

Stir in the balsamic vinegar and ⅛ tsp salt and pepper then turn the heat to medium-low. Continue gently frying the onions until very soft and sticky/sweet (approx 20 minutes).

Meanwhile, horizontally slice each chicken breast right through the centre to create 4 even-sized breasts. Season with ½ tsp salt and ¼ tsp garlic powder, thyme and black pepper.

Remove the onions from the pan and wipe out the pan if needed. Raise the heat to medium-high and add the remaining oil and butter.

Add the chicken and fry both sides for a few minutes until golden on the outside and just about cooked through the centre. Remove and place on a plate to one side then turn the heat to medium-low.

At this point preheat the grill/broiler to high.

Add the onions back to the pan and stir in the flour. Gradually pour in the stock, stirring as you go to avoid lumps, then stir in the Worcestershire sauce and the resting juices from the chicken.

Check for seasoning then add the chicken back in the pan. Spoon some of the onions then top with gruyere.

Place under the grill/broiler just until the cheese melts then remove immediately so you don't overcook the chicken.

Spoon over some more of the onions, garnish with chives then serve up and enjoy!