4 Medium-large Boneless skinless Chicken Breasts (almost 2 pounds)
Kosher Salt and ground Black Pepper
4 tablespoons Unsalted Butter divided
1 1/2 cups chopped Yellow Onion
8 ounces Cremini Mushrooms cleaned
1 cup Low Sodium Chicken Stock
1 teaspoon fresh Thyme leaves
1/2 teaspoon Crushed Red Pepper Flakes
2 tablespoons Balsamic Vinegar
1/2 cup Heavy Cream
Salt and pepper the chicken breasts. Melt 2 tablespoons of the butter in a large heavy skillet (with lid) over medium-high heat. Place the chicken breasts in the pan and cooke 4 minutes; flip and cook an additional 4 minutes.
Turn off the heat and place the lid on the skillet allowing chicken to continue to cook off heat for 10 minutes. Remove chicken from the skillet and place on a plate (note: it will continue to cook as it rests).
Melt the remaining 2 tablespoons of butter in the same skillet over medium heat. Add the onions and saute to combine with the butter and pick up loose bits from the chicken; 1-2 minutes. Add the mushrooms and stir to combine. Continue cooking, stirring periodically for 15 minutes.
Add the chicken stock, vinegar, thyme and red pepper flakes. Increase the heat to medium-high, stirring often and allow to simmer to slightly thicken; 2-3 minutes.
Lower the heat to low, stir in the cream. Add the chicken back to the skillet and spoon the sauce over the chicken and allow to warm in the sauce for a few minutes. Serve hot.