Preheat oven to 375°F. Cut a 2-inch pocket in the thickest part of each chicken-breast half. In a small bowl, combine the Feta and tapenade, then stuff ¼ of the mixture into each pocket. In a nonstick skillet, heat 2 tablespoons of the olive oil on medium to high heat. Season the chicken with the pepper and add it to the skillet, cooking 2 minutes on each side or until brown.Transfer to an oven-safe baking dish drizzled with 1 tablespoon of the olive oil and bake for 18 minutes. When the chicken is 5 minutes from being finished, put the remaining olive oil and the salsa in the skillet and heat through. Add the spinach and cookuntil slightly wilted. Slice the chicken and serve it with asqueeze of lemon juice and the capers, if desired, with the spinach on the side.