Preheat oven to 375 degrees.

Take a pairing knife and gently pierce the thickest part of the top of the chicken breast. Push it through the chicken as far as it will go. Gently use the knife to make a pocket in the chicken, but don't cut all the way through. Place the chicken breasts between two pieces of plastic wrap. Pound out the chicken slightly so it's about ¾-inch thick. Set aside.

Add the spinach to a large cast-iron skillet. Turn the heat on medium. Use tongs to toss the spinach until it’s completely wilted. Set spinach aside and wipe the pan clean.

Add the cooked spinach, feta, nutmeg, egg yolk, onion, garlic, and salt to a small bowl. Mix to combine.

Put the spinach and feta filling in a plastic baggie. Cut the end off. Gently fill the pocket with your filling, if needed use fingers to nestle the filling into the pocket. Season the chicken on both sides with plenty of salt and pepper.

Heat a large cast-iron skillet to a medium-high heat. Add 2 teaspoons of butter and 1 teaspoon of olive oil. Once the oil and butter are hot, add the chicken breasts. Sear on both sides until they are golden brown and have formed a crust, about 4-5 minutes per side.

Remove the chicken from the skillet and add the white wine. Use a wooden spoon to scrape up the brown bits from the bottom of the pan. Simmer the wine until reduced by half. Add lemon juice and remaining butter. Add the chicken back to the pan. Put the chicken in the oven and bake until cooked through, about 15 minutes.