Heat a large skillet over medium heat. Add 1 tablespoon olive oil to pan. Add onion; cook 8 minutes, stirring frequently. Remove pan from heat; stir in dill, spinach, and feta cheese. Cool 10 minutes. Cut a horizontal slit through the center of each chicken breast to form a pocket. Stuff each pocket evenly with spinach mixture. Close pockets with toothpicks. Sprinkle with salt and pepper. Heat skillet over medium-high heat. Add 1 1/2 teaspoons olive oil to pan. Add chicken; cook 4 minutes. Turn chicken. Cover pan, reduce heat to medium, and cook 5 minutes or until chicken is done.