Trim and pat dry chicken breasts.
Pound chicken breasts to an even thickness, about ½" thick.
Season on both sides with 1 teaspoon kosher salt and ½ teaspoon black pepper.
Add olive oil to a skillet over medium-high heat. After a minute or so, sprinkle a drop of water in the pan. If the water sizzles, the oil is ready to brown the chicken.
Add your seasoned chicken breasts to the pan. Don’t crowd the pan; cook in batches if necessary.
Turn after five minutes and brown the other side for another five minutes.
Remove the chicken and set aside. Do not clean your pan.
Leave the skillet over the same heat and add butter. While the butter melts, you can use a wooden spoon to scrape up the brown bits and pieces that are stuck to the bottom of the pan.
Add onions and stir regularly for a couple of minutes until the onions become translucent and absorb the butter.
Add the roughly chopped mushrooms and the minced garlic. Watch the heat and turn it down a bit if it’s too hot. The mushrooms should sizzle a little.
Once your mushrooms release some of their moisture, after about 5 minutes, add chicken stock and cook for another 10 minutes.
Add heavy cream, balsamic vinegar, and thyme.
Bring to a boil for a couple of minutes to reduce some of the liquid. Let it bubble until it reaches your desired thickness.
Now it’s time to taste. Depending on what type of butter and what type of chicken stock you used, it might need a little more salt.
When the balsamic mushroom sauce is ready to go, add your chicken back into the pan, sprinkle it with a handful of chopped flat-leaf Italian parsley, and cook for a couple of minutes. Just enough to reheat the chicken.
Serve with your favorite side dish and enjoy!