Slice chicken breasts horizontally and season with salt and black pepper.
Dredge chicken lightly in all-purpose flour, shaking off excess.
Heat olive oil and butter in a large skillet over medium heat.
Cook chicken until golden on both sides, then remove and set aside.
Add mushrooms to the skillet and sauté until browned.
Stir in minced garlic and cook until fragrant.
Pour in Marsala wine and simmer until reduced by half.
Add chicken broth and stir to combine.
Return chicken to the skillet and simmer for 10 minutes.
Stir in heavy cream if using and cook until sauce thickens.
Garnish with fresh parsley and serve warm.