Here is one simple technique that guarantees juicy results every time. First, pound the chicken: making it thinner ensures quick and even cooking. Next, dredge the chicken lightly in seasoned flour, which helps it turn deep golden brown when sauteed. After the chicken is cooked, deglaze the skillet with broth, wine or a little water to make a pan sauce. The liquid loosens the flavor-boosting browned bits from the bottom of the pan, making a tastier sauce.
- Trim visible fat from chicken breasts. Remove the tenders (the long thin flaps); reserve for another use. Place trimmed chicken breasts between 2 pieces of plastic wrap. Pound with a rolling pin, meat mallet or heavy skillet until flattened to an even thickness, about 1/2 inch. Combine flour, salt and pepper in a shallow glass dish. Dredge chicken in seasoned flour, shaking off excess. (Discard any leftover flour.)
- Heat oil in a large nonstick skillet over medium-high heat. Add the chicken and cook until well browned and no longer pink in the center, 4 to 5 minutes per side. Transfer to a plate, cover and keep warm.
- Add mushrooms and leek to the pan; cook over medium heat, stirring often, until most of the moisture has evaporated and the mushrooms are beginning to brown, 4 to 6 minutes. Add wine (or vermouth) and deglaze, scraping up any browned bits, for about 1 minute. Mix broth and cornstarch in a small bowl. Add to the pan and cook until thickened, 2 to 3 minutes. Stir in sour cream and mustard; cook until heated through, about 1 minute. Season with salt and pepper. Spoon over the chicken and sprinkle with parsley.