01 - Place chicken breasts between two sheets of plastic wrap and gently pound to an even 1/2-inch thickness.
02 - In a shallow bowl, mix flour, salt, and pepper. Dredge each chicken breast in the flour mixture, shaking off excess.
03 - Heat olive oil and butter in a large skillet over medium heat. Add chicken breasts and cook for 4-5 minutes per side until golden and cooked through. Transfer to a plate and cover loosely with foil.
04 - In the same skillet, add mushrooms and shallot. Sauté for 4-5 minutes until mushrooms are browned and shallots are softened. Add garlic and cook for 30 seconds more.
05 - Pour in chicken broth and white grape juice. Stir in lemon juice and Dijon mustard, scraping up any browned bits from the bottom. Bring to a simmer.
06 - If desired, mix cornstarch and water in a small bowl, then stir into the sauce. Simmer for 2-3 minutes until slightly thickened.
07 - Return chicken to the skillet and simmer in the sauce for 3-4 minutes, spooning sauce over the chicken.
08 - Remove from heat, sprinkle with chopped parsley, and serve with lemon wedges if desired.