Drain the spinach. Place the spinach into a clean (but not precious) kitchen towel and twist to remove the excess water.
Make the stuffing. Add the spinach to a medium mixing bowl along with ½ cup of chopped walnuts (save ¼ cup walnuts for later). Add the sun dried tomatoes, parsley, garlic, ricotta, mozzarella, and parmesan. Season with the oregano and a pinch of salt and pepper. Drizzle with a little olive oil, and mix until well combined.
Prep the chicken. Pat the chicken breasts dry and season on both sides with salt and pepper. Now, you need to make a pocket in each chicken breast. Lay a chicken breast flat on a cutting board. Place a hand firmly on top of the breast. Insert a good chef’s knife in the thickest part of the breast and slice to make a pocket about 3- to 4-inches wide (only cut 3/4 of the way in or you will split the chicken breast).
Stuff the chicken breasts. Spoon the stuffing mixture into the pocket of each chicken breast (try to equally divide the stuffing among all four pieces).
Sear the chicken. In a large cast iron skillet, heat 3 tablespoons olive oil over medium-high heat until shimmering but not smoking. Add the chicken in and cook on one side for 5 to 7 minutes, then turn over and cook on the other side for another 5 minutes or so (chicken’s internal temperature should register 165°F).
Finish and serve. Remove from heat and allow the chicken 3 to 5 minutes to rest. If you like, slice the chicken into smaller pieces. Garnish with the remaining walnuts, a bit of parsley and sun dried tomatoes. Enjoy!