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FOR THE CHICKEN: Use tip of sharp chefs knife to cut each breast horizontally, starting at thinnest end and stopping knife tip 1/2 inch away from edge so that halves remain attached (butterfly).
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Open up breasts to create 4 cutlets.
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Place 1 cutlet at a time in heavy-duty zipper-lock bag and pound to 1/4-inch thickness (cutlet should measure about 8 inches by 6 inches).
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Trim about 1/2 inch from long sides of cutlets (about 1 1/2 to 2 ounces of meat per cutlet, or a total of 1/2 cup from all 4 cutlets) to form rectangles that measure about 8 by 5 inches.
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Process all trimmings in food processor until smooth, about 20 seconds.
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Transfer puree to medium bowl and set aside.
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(Do not wash out food processor bowl.
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).
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FOR THE STUFFING: Heat 1 tablespoon oil in 12-inch skillet over medium-high heat until shimmering.
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Add mushrooms and cook, stirring occasionally, until all moisture has evaporated and mushrooms are golden brown, 8 to 11 minutes.
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Add 1 tablespoon oil and leek; continue to cook, stirring frequently, until softened, 2 to 4 minutes.
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Add garlic and thyme, and cook, stirring frequently, until fragrant, about 30 seconds.
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Add 1 1/2 teaspoons lemon juice and cook until all moisture has evaporated, about 30 seconds.
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Transfer mixture to bowl of food processor.
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Return pan to heat; add wine and scrape pan bottom to loosen browned bits.
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Transfer wine to small bowl and set aside.
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Rinse and dry skillet.
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Pulse mushroom mixture in food processor until roughly chopped, about five 1-second pulses.
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Transfer mushroom mixture to bowl with pureed chicken.
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Add 1 1/2 teaspoons parsley, 3/4 teaspoon table salt, and 1/2 teaspoon pepper.
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Using rubber spatula, fold together stuffing ingredients until well combined (you should have about 1 1/2 cups stuffing).
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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.
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Roll each breast up as tightly as possible without squeezing out filling and place seam-side down.
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Evenly space 3 pieces twine (each about 12 inches long) beneath each breast and tie, trimming any excess.
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Season chicken with salt and pepper.
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Heat remaining tablespoon oil in skillet over medium-high heat until just smoking.
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Add chicken bundles and brown on 4 sides, about 2 minutes per side.
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Add broth and reserved wine to pan and bring to boil.
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Reduce heat to low, cover pan, and cook until instant-read thermometer registers 160 degrees when inserted into thickest part of chicken, 12 to 18 minutes.
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Transfer chicken to cutting board and tent loosely with foil.
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TO MAKE SAUCE AND SERVE: While chicken rests, whisk mustard into cooking liquid.
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Increase heat to high and simmer, scraping pan bottom to loosen browned bits, until dark brown and reduced to 1/2 cup, 7 to 10 minutes.
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Off heat, whisk in butter and remaining 1 1/2 teaspoons parsley and 1 1/2 teaspoons lemon juice; season with salt and pepper.
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Remove twine and cut each chicken bundle on bias into 6 medallions.
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Spoon sauce over chicken and serve.
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Tip: Removing Chicken Tenderloins: The chicken tenderloin is the small piece of meat attached to the underside of each breast half.
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Many recipes for boneless skinless chicken call for removing the tenderloin before cooking.
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Here's how to do it.
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Lay the breast on the work surface smooth-side down and use one hand to locate the tenderloin.
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Place one hand flat against the chicken breast and hold it against the board while you gently pull the tenderloin off with your other hand.
