Stuffed Chicken Breasts

1. FOR THE CHICKEN: Slice each chicken breast horizontally, stopping 1⁄2 inch from edges so halves remain attached. Pound between 2 sheets of plastic wrap to even 1⁄4-inch thickness (each cutlet should measure about 8 by 6 inches) with meat pounder. Trim about 1⁄2 inch from long sides of cutlets (about 1-1⁄2 to 2 ounces of meat per cutlet, or total of 1⁄2 cup from all 4 cutlets) to form rectangles that measure about 8 by 5 inches. Transfer chicken cutlets to plate, cover, and refrigerate while making stuffing.
2. Process all trimmings in food processor until smooth, about 20 seconds. Transfer puree to medium bowl and set aside. (Do not wash food processor bowl.)
3. Heat 1 tablespoon oil in 12-inch skillet over medium-high heat until shimmering. Add mushrooms and cook, stirring occasionally, until softened and lightly browned, 8 to 10 minutes. Stir in 1 tablespoon oil and leek and cook, stirring often, until leek is softened, 2 to 4 minutes. Stir in garlic and thyme and cook until fragrant, about 30 seconds. Stir in 1-1⁄2 teaspoons lemon juice and cook until all moisture has evaporated, about 30 seconds. Transfer mixture to food processor. Add wine to now-empty skillet, scraping up any browned bits. Transfer wine to small bowl and set aside. Rinse and dry skillet.
4. Pulse mushroom mixture in food processor until roughly chopped, about 5 pulses. Transfer mushroom mixture to bowl with pureed chicken. Add 1-1⁄2 teaspoons parsley, 3⁄4 teaspoon salt, and 1⁄2 teaspoon pepper. Using rubber spatula, fold together stuffing ingredients until well combined (you should have about 1-1⁄2 cups stuffing).
5. TO ASSEMBLE AND COOK: With thinnest ends of cutlets pointing away from you, spread one-quarter of stuffing evenly over each cutlet with rubber spatula, leaving 3⁄4-inch border along short sides of cutlet and 1⁄4-inch border along long sides. Roll each breast up as tightly as possible without squeezing out filling and place on plate seam side down. Evenly space 3 pieces of kitchen twine (each about 12 inches long) beneath each breast and tie, trimming any excess.
6. Season chicken with salt and pepper. Heat remaining 1 tablespoon oil in skillet over medium-high heat until just smoking. Add chicken bundles and brown on all sides, about 8 minutes per side. Add broth and reserved wine to skillet and bring to boil. Reduce heat to low, cover, and cook until chicken bundles register 160 degrees, 12 to 18 minutes. Transfer chicken to carving board and tent loosely with aluminum foil.
7. FOR THE SAUCE AND TO SERVE: While chicken rests, whisk mustard into cooking liquid, scraping up any browned bits. Bring to simmer over high heat and cook until dark brown and measures 1⁄2 cup, 7 to 10 minutes. Off heat, whisk in butter, 1 piece at a time, remaining 1-1⁄2 teaspoons lemon juice, and remaining 1-1⁄2 teaspoons parsley. Season with salt and pepper to taste. Remove twine and cut each chicken bundle on bias into 6 medallions. Spoon sauce over chicken and serve.

STUFFING CHICKEN BREASTS
1. Slice each breast horizontally, stopping 1⁄2 inch from edges so halves remain attached.
2. After pounding each butterflied chicken breast to uniform 1⁄4-inch thickness, trim about 1⁄2 inch from long sides of each cutlet to form 8 by 5-inch rectangle.
3. Spread stuffing evenly over each cutlet, leaving 3⁄4-inch border along short sides and 1⁄4-inch border along long sides.
4. With short side facing you, roll up each cutlet and secure it snugly with kitchen twine.