In a medium mixing bowl, blend softened cream cheese, shredded mozzarella, grated Parmesan, minced garlic, chopped baby spinach, and red pepper flakes with a fork until thoroughly combined. Season with 1/4 teaspoon sea salt.

Pat the chicken breasts thoroughly dry with paper towels. Place each breast flat on a cutting board, place your palm firmly on top, and use a sharp chef’s knife to carefully cut a deep horizontal pocket into the thickest side (about 3/4 of the way through, taking care not to slice entirely through the back or edges).

Generously divide and spoon the spinach-cheese filling into each chicken pocket. Press the edges together to close, and secure each seam with 2 to 3 wooden toothpicks inserted horizontally.

In a small dish, mix together the smoked paprika, Italian seasoning, remaining 3/4 teaspoon salt, and black pepper. Season both outer sides of the stuffed chicken breasts evenly.

Heat olive oil and butter in a large heavy-bottomed cast-iron skillet over medium-high heat. Once hot and shimmering, carefully lay the stuffed chicken breasts in the skillet. Sear undisturbed for 5 to 6 minutes until a rich golden-brown crust forms.

Using tongs, gently flip the chicken breasts. Lower heat to medium, cover the skillet with a lid, and cook for another 7 to 9 minutes until the internal temperature reaches 165°F (74°C) on an instant-read thermometer. Transfer chicken to a clean plate and tent loosely with foil to rest.

Keep the skillet over medium heat. Pour in the chicken bone broth, using a wooden spoon to scrape up all the caramelized golden fond from the skillet bottom. Simmer for 2 minutes until reduced by half.

Reduce heat to low. Stir in the heavy cream and lemon juice, simmering gently for 2 minutes until a silky pan sauce forms. Remove toothpicks from the chicken, return breasts to the skillet to spoon the warm velvety sauce over the top, garnish with fresh parsley, and serve immediately.