Pat the chicken breasts dry with paper towels, then lightly season both sides with salt and pepper.
Use a sharp knife to carefully slice a horizontal pocket into the thick side of each chicken breast, being mindful not to cut all the way through.
In a small bowl, mix chopped herbs, lemon zest, minced garlic, chopped spinach (if using), and shredded cheese. Season with a pinch of salt and pepper.
Stuff each chicken pocket with the prepared herb and cheese mixture, pressing gently to pack the filling in.
Heat olive oil in a skillet over medium-high heat until shimmering and slightly smoking.
Place the stuffed chicken breasts seam-side down in the skillet and cook for about 6-7 minutes until golden brown and seared.
Flip the chicken breasts carefully using tongs and cook the other side for another 6-7 minutes, ensuring an even sear.
Check the internal temperature with an instant-read thermometer; it should reach 74°C (165°F). If not, cook for a few more minutes.
Once cooked through, transfer the chicken to a plate and tent loosely with foil. Let it rest for 5 minutes to retain juices.
Slice the chicken breasts crosswise to reveal the juicy, flavorful filling inside.
Serve the sliced chicken with a squeeze of fresh lemon juice and drizzled pan juices for added brightness.
