Preheat the oven to 325 °F.
Prepare the chicken. Pound out each chicken breast with the flat side of a meat mallet or with the end of the knife. Add flour to a shallow bowl and season it with a good amount of salt and freshly ground pepper, and mix with the fork.
Sip the chicken in the seasoned flour until it’s well coated. Alternatively, sprinkle some salt and pepper on each side of the breast and then dip them in the flour.
Brown the chicken. Heat 2 tablespoons of butter and olive oil in a pan over medium-high heat. Cook the chicken breasts for about 3-5 minutes on each side, until golden brown. Set the chicken breasts aside once cooked.
Cook the mushrooms. Add the whole garlic cloves, sliced mushrooms, and thyme sprigs to the same pan. Sauté over medium heat for about 5minutes until the mushrooms are soft and the garlic is fragrant.
Add the sherry to deglaze and let it simmer to thicken up. Scrape the bottom on the pan, add 3 cloves of the chopped garlic and cook until fragrant. Add Marsala wine, chicken stock, salt, and pepper to taste. Let the sauce simmer until it thickens and all alcohol evaporates, leaving just the delicious flavors behind.
Return the chicken to the pan and cover it with a lid. Bake in the oven for about 30 minutes until the chicken is fully cooked.
Take the pan out of the oven and remove the chicken and thyme springs. Add ¼ cup of flour to the butter and mix. Then add this mixture to the marsala sauce and let it simmer over medium heat for about 5 minutes. Once it thickens return the chicken to the pan and coat with sauce. Professional Ina Garten’s Chicken Marsala is ready! Garnish with chopped parsley before serving!