Clean and thickly slice the mushrooms making 1 1/2 cups.
Melt 1 tablespoon of butter in a large frying pan.
Add the mushrooms to the frying pan, sprinkling them evenly with 1 teaspoon of onion powder.
Cook the mushrooms until brown and soft, stirring regularly. This takes about 5 minutes.
Remove the mushrooms and set aside.
Add one tablespoon of olive oil to the frying pan.
Add the chicken breasts to the olive oil. Turn regularly and cook over medium high heat until it is browned and no longer pink on the inside, about 8 minutes.
Remove the cooked chicken and return the mushrooms to the pan.
Add the marsala and let it cook over medium high heat, reducing the wine by about half. This takes about 4-5 minutes.
Return the chicken to the frying pan.
Add the chicken broth and let simmer lightly.
Pour the heavy cream into the mixture while stirring/whisking continually. If you don't keep stirring, the cream may curdle.
Once everything is heated through, use a ladle to scoop out one cup of liquid, placing it into a heat resistant cup (I used a measuring cup). Lower the heat under the frying pan so the rest of the liquid doesn't evaporate.
Add one tablespoon of flour to the liquid. Mix it in with a fork, making sure to break up any lumps.
Once completely mixed, return the liquid to the pan, stirring. Continue to heat everything through over medium heat.
Season with salt and pepper to taste.