Preheat your oven to 375°F (190°C).
Heat the olive oil in a skillet over medium heat and sauté the garlic for 1 minute until fragrant.
Add the chopped spinach to the skillet and cook until wilted, about 2–3 minutes.
Remove the skillet from heat. Stir in the crumbled feta cheese, lemon juice, and season with salt and pepper to taste.
Using a sharp knife, create a pocket in each chicken breast without cutting all the way through.
Stuff each chicken breast with the spinach and feta mixture, using toothpicks to secure the openings if necessary.
Season the outside of the chicken breasts with salt and pepper.
Place the stuffed chicken breasts in a baking dish and cover with foil.
Bake in the preheated oven for 25–30 minutes, or until the chicken reaches an internal temperature of 165°F (75°C).
Remove the foil during the last 5 minutes for a golden finish, if desired.
Let the chicken rest for 5 minutes before slicing and serving.
