Instructions

1. Thaw 16 ounces frozen spinach according to package directions. Transfer to a clean kitchen towel, roll it up in the towel, and squeeze out all excess moisture. Transfer to a food processor fitted with the blade attachment and push it to one side a little.
2. Coarsely chop 1 medium sweet or yellow onion and add to the food processor. Add 3 to 4 ounces prosciutto and 1/8 teaspoon freshly grated nutmeg, if using. Process until the mixture is like a thick pesto sauce, smooth, and a bit sticky, stopping and using a rubber spatula to scrape down the sides as needed.
3. Cut 16 (12-inch long) pieces of kitchen twine. Pound 4 chicken breasts: Working with 1 breast at a time, place inside a gallon zip-top bag and seal the bag, pressing out as much air as possible. Pound with the flat side of a meat mallet, rolling pin, or small skillet to an even 1/4-inch thickness. Set the flattened chicken breast aside and repeat with remaining breasts.
4. Arrange the chicken on a cutting board smooth-side down and season with 1 teaspoon kosher salt and 1/2 teaspoon black pepper. Halve each breast lengthwise. Spread about 2 1/2 tablespoons of the spinach filling onto each piece, leaving a bit of a border around the edges. You will have filling leftover, and that will be added to the sauce later.
5. Starting at a short end, roll up each piece tightly into a coil. Tie 2 pieces of kitchen twine around each cutlet to secure. Cut off any extra twine. Transfer the roll ups to a large plate.
6. Melt 2 tablespoons unsalted butter in a 10-inch high-sided skillet or Dutch oven over medium heat. Add half the roll ups and sear until browned on a few sides, 6 to 8 minutes total. Return to the plate and sear the remaining roll ups.
7. Add 1 1/2 cups dry white wine to the pan and scrape up the browned bits from the bottom with a wooden spoon. Return the chicken and any accumulated juices to the pan. Add more wine if needed to come about 3/4 of the way up the sides of the chicken.
8. Cover with the lid just slightly ajar and simmer for 30 minutes. Uncover, add the remaining spinach filling, and gently stir to combine. Flip the chicken, cover again with the lid slightly ajar, and simmer for 15 minutes more.
9. Using tongs, transfer the chicken to a clean cutting board or plate. Cut and remove all the twine. Add 1 1/2 cups heavy cream to the sauce and bring to a simmer. Return the chicken and any accumulated juices to the pan and simmer until the flavors meld, 5 to 10 minutes. Taste and sauce and season with kosher salt as needed. Serve the chicken and sauce over cooked rice pilaf, egg noodles, or pasta if desired.