Place a medium skillet on medium heat. (If serving with starch, start cooking that separately now.)
Add olive oil followed by chicken breasts. Cook 2-3 minutes per side until golden on the outside and cooked through. If your chicken breasts are larger, you may need an extra minute or two.
Remove chicken to a plate. (Optional: place in a 200°F oven to keep warm.)
Raise the heat to medium-high and add mushrooms and sauté for 3-5 minutes until browned, stirring regularly.
Add marsala wine to the pan and allow to boil for a minute to let the alcohol evaporate.
Stir in chicken broth, heavy cream, garlic powder, mustard powder, salt and pepper. Bring to a boil.
Reduce heat to medium-low and simmer for 7-10 minutes until the sauce is thick enough to coat a spoon.
Place chicken back in the pan along with the optional tomatoes. Spoon sauce on top and simmer for 1-2 minutes more.
Remove to plates and serve with optional starch. Sprinkle optional minced parsley on top. Serve and enjoy!