Pat the chicken dry and sprinkle with the garlic powder and some salt & pepper. Add oil to a large skillet and turn the heat to medium-high.
Allow it to become hot, add the chicken and cook it for about 6 to 12 minutes or until golden brown.
Transfer the chicken onto a clean plate. In the same skillet, add butter and onion.
Sauté the onion until it's softened and lightly browned (about 10-15 minutes, depending on how thick you sliced the onion), stirring them as needed to prevent from sticking.
Add balsamic vinegar, red pepper flakes, thyme, and sugar. Stir until well mixed. Simmer for a few minutes to cook the vinegar.
Stir in the chicken broth and scrape up any brown bits from the bottom of the pan.
Stir in the heavy cream until you've got a smooth sauce, and let it heat through for a couple of minutes (ensure it doesn't bubble too much or else it could curdle).
Add the chicken back to the skillet and spoon some sauce over top. Season with extra salt & pepper if needed, and serve immediately with some chopped parsley sprinkled over top.