Cook sausage, oregano and garlic in large skillet over MEDIUM-HIGH heat 8-10 minutes or until sausage is thoroughly cooked, stirring frequently; drain. Return sausage mixture to skillet. Stir in lemon juice.
Combine spinach, tomatoes, cheese, olives and pepper in large bowl.
Fill pita halves with sausage mixture, then spinach mixture. Add onion slices, if desired.