In a ziptop bag, combine the lemon juice, olive oil, Greek seasoning, salt and pepper. Cut a slit through the middle of each chicken breast, being careful not to cut all the way through. Add to ziptop bag and squish around until thoroughly coated. Place in refrigerator and marinate for 30 minutes.
Preheat oven to 350 degrees F. Meanwhile, combine feta, spinach (squeeze dry in a kitchen towel to get all the excess liquid out) and pepper. Remove chicken from marinade and divide the filling evenly between each chicken breast. Fill slit with filling and secure closed with toothpicks.
Heat a skillet over medium heat and add a drizzle of olive oil. Sear chicken for 3 to 4 minutes per side. Place pan in preheated oven and cook about 20 minutes, or until internal temperature reaches 165 degrees F. Remove toothpicks and serve.