Lightly season the chicken breasts with sea salt and black pepper on both sides. In a large sauté pan over medium-low heat, add 2 tablespoons of the butter and the onions. Cook until the onions are nice and caramelized, about 20 minutes. Remove the pan from the heat and set aside. In a separate pan, over medium-high heat, pan-sear the chicken breasts in the remaining 3 tablespoons of butter. Brown the chicken on both sides and then remove from pan and set aside. (Chicken will not be fully cooking during this stage)
De-glaze the pan with chicken stock. Using a rubber spatula, scrape the pan and mix in any remaining bits of chicken. Let simmer for 5 minutes.
Reduce the heat to low, add the heavy cream, balsamic vinegar, mushrooms and a little sea salt and black pepper. Let simmer for 10 minutes.
Add the chicken breasts back to the sauce pan, and simmer until the chicken is cooked all the way through, about 12 minutes
Remove the chicken breasts from the pan and plate. Add the Parmesan cheese and caramelized onions to the sauce and stir until the cheese is melted into the sauce. Pour the sauce over top of chicken.
Garnish with fresh parsley, if using.
Per Serving – Calories: 482 | Fat: 33g | Protein: 42g | Net Carbs: 6.5g