Preheat oven to 375F. Heat oil in a Dutch oven over medium heat. Add onion, celery, oregano, garlic, and tempeh, and saute 5 minutes. Combine cornstarch and water, and stir well. Add cornstarch mixture and milk to pan; bring to a boil. Reduce heat, and simmer 20 minutes, stirring occasionally. Stir in lemon juice, dill, salt, pepper, macaroni, 4 ounces feta, and spinach. Spoon mixture into a 13 x 9-inch baking dish coated with cooking spray; sprinkle with 4 ounces feta. Bake at 375F for 30 minutes or until lightly browned.