To a mixing bowl, add the cream cheese, gruyère, mozzarella, Parmesan, spinach, garlic powder, nutmeg, salt, and a few grinds of black pepper.
Stir to combine well, cover, and refrigerate.
Step 3
Prepare the chicken. Place each breast in turn on a large cutting board, and press gently on the top of each piece with your non-dominant hand.
Using your dominant hand, cut a deep, wide slit lengthwise in each chicken breast with a sharp chef’s knife, leaving about ¼ inch (about 5mm) on each side, creating a pocket.
Step 4
When you have prepared a pocket in each piece of chicken, remove the cheese mixture from the refrigerator. Stuff each piece of chicken generously with the cheese mixture, leaving space at the opening to close the pocket.
Using toothpicks or a medium-sized wooden skewer, weave the pocket shut as if you were sewing. Alternatively, you can use kitchen twine.
Step 5
Sprinkle each breast on both sides with salt, pepper, and paprika.
Step 6
Heat the olive oil in a large, heavy, ovenproof skillet over medium-high heat. Add the chicken to the pan. Sear for 3-5 minutes on each side, or until well-browned. Do not worry if filling spills out into the pan.
Step 7
Transfer the skillet to the oven, and bake until fully cooked—the temperature of both the chicken and filling should register 165ºF (75ºC) before serving. This may take as little as 20 minutes for a thinner breast, and up to 40 for a thicker one.