Cook sausage in a large skillet over medium high heat, stirring frequently, until thoroughly cooked and no longer pink. Stir in garlic and oregano; cover and cook 1 minute; remove from heat. Stir in ricotta and lemon juice. In a separate bowl, combine tomatoes, spinach, feta cheese, olives and optional black pepper; stir well. Spread sausage mixture evenly inside each pita half, dividing mix evenly among the eight halves; spread 1/4 cup tomato-spinach mixture evenly across sausage mixture. Add red onion slices if desired. Serve immediately.