Preheat oven to 350 degrees. Heat a large, oven safe skillet over medium-low heat. Add butter and allow it to melt. Add onions and stir to coat in the butter. Season with 1 1/2 teaspoons salt and 1 teaspoon black pepper. Stir again. Cook for 25 minutes, stirring every 5 minutes.
Add beef stock, worcestershire sauce and thyme to the onions. Stir to coat. Cook another 10 minutes, stirring occasionally. Remove the onions from the skillet and return it to the stove top.
Season the chicken with 1 teaspoon salt, 1/2 teaspoon black pepper, garlic powder and onion powder. Add to the skillet the onions were cooked in. Sear chicken for 2 minutes per side. Turn off heat.
Top chicken with the caramelized onions then place a slice of cheese on each one. Place in the preheated oven and bake for 15 minutes, or until chicken is cooked through.