Season the chicken: Pat the chicken breasts dry with paper towels. Season both sides generously with salt and pepper.
Sear the chicken: Heat 1 tablespoon of olive oil in a large skillet over medium high heat. Add the chicken breasts and cook for 5 minutes per side, until golden brown and cooked through (internal temperature reaches 165°F). Remove the chicken from the skillet and set aside on a plate. Cover loosely with foil to keep warm.
Cook the mushrooms: In the same skillet, heat the remaining 1 tablespoon of olive oil. Add the halved button mushrooms and cook for 3-4 minutes, stirring occasionally, until slightly golden brown.
Add the garlic: Add the crushed garlic to the skillet. Sauté for 30-60 seconds until fragrant, being careful not to burn the garlic.
Make the cornstarch slurry: In a small bowl, whisk together the cornstarch and 2 tablespoons of water until fully dissolved and smooth.
Build the sauce: Pour the chicken broth and the cornstarch slurry into the skillet. Season with 1/2 teaspoon salt and 1/4 teaspoon pepper. Bring the liquid to a bubble, stirring occasionally.
Add the cream cheese: Reduce the heat to medium low. Add the softened cream cheese to the skillet. Whisk continuously until the cream cheese is fully melted and the sauce is smooth and thickened, about 2-3 minutes.
Combine and serve: Return the seared chicken breasts to the skillet, spooning the creamy mushroom sauce over the top. Let everything heat through for 1-2 minutes. Garnish with fresh chopped parsley. Serve warm.