Preheat oven to 400°F (200°C) and lightly grease a 9x13 inch baking dish.
Heat 1 tablespoon of olive oil in a skillet over medium heat; sauté chopped spinach and minced garlic until the spinach is fully wilted.
Remove spinach from heat and drain any excess moisture, then combine with crumbled feta cheese in a small mixing bowl.
Using a sharp knife, slice a pocket into the thickest part of each chicken breast, ensuring you do not cut all the way through to the other side.
Evenly distribute the spinach and feta mixture into the pockets of the four chicken breasts and secure the openings with wooden toothpicks if necessary.
Coat the exterior of the chicken breasts with the remaining 1 tablespoon of olive oil and season with oregano, salt, pepper, and red pepper flakes.
Place the stuffed chicken breasts into the prepared baking dish and drizzle with lemon juice.
Bake for 20 to 25 minutes until the chicken reaches an internal temperature of 165°F (74°C).
Remove from oven and let the chicken rest for 5 minutes before removing toothpicks and serving.