Place the chicken breasts between sheets of parchment paper and gently pound them to an even thickness if needed.
Season both sides of the chicken with salt and black pepper.
Heat 1 tablespoon of olive oil in a large skillet over medium heat. Add the chicken and cook until lightly golden and cooked through, about 4–6 minutes per side depending on thickness. Transfer the chicken to a plate.
Add the remaining tablespoon of olive oil to the same skillet. Add the sliced mushrooms and sauté for 1–2 minutes until they begin to soften and brown.
Pour in the white grape juice and lemon juice. Increase the heat to medium-high and let the mixture simmer for 2–3 minutes, stirring occasionally.
Add the chicken broth, heavy whipping cream, ground mustard, and garlic powder. Stir well to combine.
Bring the sauce to a gentle boil, then reduce the heat and simmer for about 8–10 minutes, or until the sauce thickens slightly.
Return the cooked chicken to the skillet and spoon the creamy sauce over the top.
Simmer for another 1–2 minutes so the chicken can warm through and absorb some of the sauce.
Garnish with chopped fresh parsley and serve immediately. This creamy chicken marsala is especially delicious with pasta, mashed potatoes, or rice.