Preheat the oven to 400°F (200°C).
Pre-soak some toothpicks in water for 10-15 minutes.
Peel and finely dice the onion. Peel and press the garlic. Cook the frozen spinach as per package instructions – I cooked mine for 4 minutes covered in the microwave.
Heat a drizzle of olive oil in a skillet over medium heat and sauté the onions until translucent. Add the fresh garlic and sauté for 30 seconds until fragrant. Add the spinach (squeezed well of any excess moisture) and cook for another minute or so. Let the mixture cool slightly while preparing the chicken.
Pat each chicken breast dry with a paper towel. Place a chicken breast flat on a cutting board and use a sharp knife to carefully cut a deep pocket into the thickest part of the breast, slicing horizontally. Leave an edge of about 1/3 inch. Try to keep the opening smaller, so the ricotta filling stays inside. If your chicken breasts are very thick place them between two pieces of plastic wrap on the cutting board and gently pound them with a meat mallet to an even thickness before slicing the pocket.
In a large mixing bowl, add the ricotta cheese, cooled spinach mixture, honey, salt, and black pepper, and stir everything together with a wooden spoon until well combined. Taste the cheese mixture and adjust the seasoning as needed.
Season the outside of each chicken breast generously with salt and freshly ground black pepper. Then spoon the spinach ricotta filling generously into each pocket. Press the sides of the chicken gently together to close the opening. Secure with two toothpicks to hold the filling in place.
Heat the olive oil in a cast iron skillet over medium-high heat. Place the ricotta-stuffed chicken breasts into the hot skillet and sear for 3 to 4 minutes per side without moving them, until a beautiful golden brown crust forms. This step locks in flavor and gives your chicken that gorgeous exterior.
Transfer the skillet to the preheated oven (or move the seared chicken to a lightly greased baking dish). Bake for 12–18 minutes, depending on the thickness of your chicken breasts, until perfectly cooked. If using a thermometer, cook until the internal temperature reaches 165°F (74°C) in the thickest part. If some of the delicious ricotta filling melts out into the pan, just spoon it back over the chicken before serving!
Remove the baked chicken with ricotta cheese from the oven and let it rest for 5-10 minutes before slicing or serving. Remove any toothpicks before serving. Enjoy!