Pat the chicken breasts dry with paper towels, then season generously with salt and pepper on both sides.
Using a sharp knife, carefully slice a horizontal pocket into the thick side of each breast, being mindful not to cut all the way through.
Fill each pocket with chopped herbs, lemon zest, cheese, and a few spinach leaves or ham slices if using, pressing gently to pack the filling inside.
Heat a tablespoon of olive oil or butter in a skillet over medium-high heat until shimmering and fragrant.
Place the stuffed chicken breasts seam-side down in the hot skillet, cooking for about 6-7 minutes until they develop a golden brown crust.
Use tongs to carefully flip the chicken breasts and cook for another 6-7 minutes, ensuring even searing on both sides.
Check the internal temperature with an instant-read thermometer; it should read 74°C (165°F) when fully cooked.
Once cooked, transfer the chicken to a plate and tent loosely with foil to rest for about 5 minutes, allowing juices to redistribute.
After resting, slice the chicken into thick medallions, revealing the flavorful filling inside.
Drizzle with pan juices or a squeeze of fresh lemon before serving for added brightness.
Enjoy your tender, juicy stuffed chicken breasts with your favorite side dishes!