Use a sharp pairing knife to cut a small slit in the thickest part of the top of the chicken. Gently use the knife (while it's in the slit) to create a pocket inside the chicken, but be sure you don't cut all the way through to the sides of the chicken. Stuff the chicken with the fontina.
Heat a large skillet to a medium heat. Add olive oil and one tablespoon of butter. Season chicken on both sides with plenty of salt and pepper. (I usually use about one teaspoon of salt per pound of chicken.) Once the butter melts, add the chicken. Sear until golden brown, about 5-6 minutes. Flip over and sear on the other side. Remove the chicken from the pan and set aside.
Add remaining butter to the pan. Add the mushrooms and onions. Sauté until softened, about 5-6 minutes.
Pour in marsala wine. Bring to a boil and reduce to a simmer. Simmer until reduced a little bit, about 2-3 minutes. Stir in chicken stock and cream. Bring to a simmer and add the chicken back to the pan. Cover and simmer until chicken is cooked through, another 6-7 minutes. The chicken is done cooking when an instant-read thermometer reaches 165 degrees.
Season sauce with salt and pepper. Garnish with chopped parsley and serve with mashed potatoes or cooked pasta.
Use a sharp pairing knife to cut a pocket in the chicken. Be sure not to cut all the way through.
Don't be shy with the cheese, but also don't overstuff the cheese into the chicken breast pocket.
When seasoning the chicken, I usually use about one teaspoon of salt per pound of chicken.
To check the doneness of the chicken, use an instant read meat thermometer, when it reaches 165 degrees you're good to plate up!