In a small bowl, whisk together cornstarch and milk until smooth. Set aside.

Place each chicken breast between two sheets of plastic wrap and pound to a 1/4-inch thickness. This ensures even cooking and tender meat.

Heat olive oil in a large nonstick skillet over medium heat. Add the chicken breasts and cook for 5-6 minutes per side, or until no longer pink in the center. Remove chicken and set aside on a plate.

In the same skillet, melt butter over medium-high heat. Add mushrooms and sliced onion, sautéing until tender and lightly browned, about 4-5 minutes.

Stir in sherry (or chicken broth), salt, and pepper. Bring the mixture to a gentle boil.

Give the cornstarch mixture another stir, then slowly pour it into the pan. Continue to cook, stirring constantly, until the sauce thickens—about 1-2 minutes.

Return the cooked chicken breasts to the skillet, spooning the sauce over them to coat. Reduce the heat and simmer for 1-2 minutes, ensuring everything is heated through.

Serve immediately, garnished with fresh parsley if desired!