Preheat oven to 400º F. Spray a 13x9 inch baking dish with non-stick spray. Set aside.

Cut each piece of chicken horizontally in half, but not all the way through. You want to be able to unfold it, stuff it with the filling and then wrap it around the filling to bake.

Season the chicken with salt, pepper, 1/2 tsp of dried thyme and 1 tablespoon of the garlic.

Heat a large skillet over medium high heat and ad the butter to melt.

Season the sliced onions with salt, pepper and the remaining dried thyme.

Add the onions to the pan can cook for 5-8 minutes, until they are nice and tender.

Reduce the heat to medium and add the rest of the garlic. Stir and cook for about 15 minutes. You want to stir them occasionally, so they don't get too browned.

Pour in the 1/4 cup of chicken broth, wine, and Worcestershire sauce. Keep stirring and cooking until the onions are browned and the sauce has thickened.

Add 2 tablespoons of caramelized onion and ¼ cup mozzarella in each chicken pocket. Seal with toothpicks.

In the bottom of a 13x9 inch baking dish mix together the rest of the chicken broth and the remaining onions. Set aside.

In the same skillet you cooked the onions in, heat the oil on medium high heat until it is nice and hot. Sear chicken breast on each side until browned. It will take about 4 minutes for each side.

Place the seared chicken over the onion mixture in the baking dish and bake for 15-20 minutes, until the chicken is cooked through.

To serve remove the toothpicks, and scoop out the onions and sauce from the baking dish.