Step 1

Butterfly the chicken breasts horizontally, stopping 1/2 inch away from one edge so the halves remain attached.

Step 2

Open up each chicken breast, cover with plastic wrap, and pound to 1/4 inch thick.

Step 3

Trim 1/2 inch from the long sides of the cutlets to form rectangles that measure about 8x5-inches. Set the trimmings aside.

Step 4

Add the trimmed chicken breast pieces to the bowl of a food processor and puree. Transfer the minced chicken to a medium bowl and set aside. You do not need to clean out the bowl o the food processor, as it will be used to puree the vegetable mixture next.

Step 5

Heat 1 tablespoon oil in a skillet over medium-high heat until shimmering. Add the mushrooms and cook, stirring occasionally, until the mushrooms are golden and the liquid has evaporated, about 10 minutes.

Step 6

Add 1 tablespoon more oil to the pan with the leek and cook until softened, stirring frequently, about 3 minutes.

Step 7

Add the garlic and thyme and cook about 30 seconds. Add 1 1/2 teaspoons lemon juice and cook for 30 seconds until the liquid has evaporated.

Step 8

Transfer the vegetables to the bowl of a food processor, and return the pan to the heat.

Step 9

Add the wine and scrape the pan to loosen browned bits,. Transfer the wine to a small bowl and set aside. Rinse and dry the skillet.

Step 10

Process the vegetables in the food processor, then transfer to the bowl containing the minced chicken. Add 1 1/2 teaspoons parsley, 3/4 teaspoon salt, and 1/2 teaspoon pepper. Fold together with a silicone spatula until well combined.

Step 11

Lay out 3 pieces of kitchen twine (approximately 12-inches long each) and 1 chicken breast cutlet, opened up, on a work surface

Step 12

Add 1/4 of the stuffing mixture evenly over the chicken, leaving a 3/4-inch border along the short sides and a 1/4-inch border along the long sides of the chicken cutlet. Roll the chicken breast up tightly without squeezing out the filling. Place seam side down on top of the 3 pieces of twine. Tie each piece of twine around the rolled chicken breast, trim excess twine if necessary. Repeat with each chicken breast.

Step 13

Heat the oven to 375 degrees. Line a baking sheet with aluminum foil and spray with nonstick cooking spray.

Step 14

Season the chicken with salt and pepper. Heat the skillet with 1 tablespoon oil over medium-high heat, just until smoking. Add the stuffed chicken and brown on all sides, about 2 minutes per side.

Step 15

Transfer the chicken bundles to the prepared baking sheet and bake until the inside registers at least 165 degrees, 20 to 30 minutes. The filling contains raw chicken, so it is important that it reaches a safe temperature in the center. While the chicken bakes, prepare the pan sauce.

Step 16

Add the broth and reserved wine to the pan.

Step 17

Whisk the mustard into the sauce and increase the heat to high and simmer until reduced to 1/2 cup, about 10 minutes.

Step 18

Remove the pan from heat and whisk in the butter, 1 1/2 teaspoons parsley, and 1 1/2 teaspoons lemon juice. Check sauce for taste and adjust with salt and pepper if needed.

Step 19

Remove the cooked chicken breasts to a cutting board and let rest for at least 5 minutes.

Step 20

Carefully remove and discard the twine.

Step 21

Slice each chicken breast through the filling into medallions. Place on a plate and spoon sauce over the chicken.