Heat 1 tbsp olive oil on medium heat in a large skillet. Add sliced Portabella mushrooms and sauté for 5-7 minutes.
Remove the mushrooms from the skillet. Add 1 tbsp olive oil to the pan and sauté the shallot for 2-3 minutes.
Add the broth and the Marsala wine to the skillet and bring to a low boil. Continue to cook on a low boil for 10-12 minutes until the sauce is reduced.
Mix the corn starch with about 1/2 cup liquid from the pan. Add the corn starch mixture back into the pan and stir with a whisk for 3-4 minutes or until the sauce just starts to thicken.
Add the mushrooms back into the pan along with the parsley and cook for 1-2 minutes. Add a pinch of black pepper.