Pour olive oil in pot over medium heat and add both types of onion. Sauté for 1 minute, and them add spinach. Sauté for approximately 10 minutes.
Preheat the broiler.
Pour spinach mixture in a bowl and cool. Add dill, eggs, feta and pepper to spinach mixture and mix very well.
Butterfly chicken breasts and fill with spinach/feta mixture. Close the breast and broil for 10 minutes on each side. After 15 minutes, douse chicken breasts with olive oil, lemon, and oregano and serve.