Preheat your oven to 375°F (190°C). Lightly brush a baking dish with a bit of olive oil and set it aside.
Using a sharp knife, carefully create a pocket in each chicken breast by inserting the blade into the thick side and sliding it horizontally to form a cavity. Try to keep the pocket from breaking through the opposite side.
In a skillet over medium heat, warm 1 tablespoon olive oil. Add garlic and sauté for about 30 seconds, just until fragrant. Add chopped spinach and cook until wilted, about 2–3 minutes. Remove from heat and let cool slightly.
In a bowl, mix the cooked spinach, feta, cream cheese or ricotta (if using), oregano, thyme, lemon zest, and a pinch of salt and pepper. Stir until well combined.
Divide the filling evenly among the chicken pockets. Gently press the edges to seal the pockets. If needed, secure with toothpicks.
Place the stuffed chicken breasts in the prepared baking dish. Brush the tops with a little lemon juice and olive oil, then season lightly with salt and pepper.
Bake for 25–30 minutes, or until the chicken reaches an internal temperature of 165°F (74°C) and the filling is hot and bubbling. If you like a golden top, switch to broil for the last 2–3 minutes, watching closely.
Let the chicken rest for 5 minutes before slicing open to reveal the creamy filling. Serve with a light side—roasted vegetables, a simple salad, or quinoa works nicely.