Melt the butter and heat the olive oil in a large pot over medium heat. Stir in the onion and garlic and cook until tender. Mix in the tomatoes, fish stock and wine. Season with oregano, salt and pepper. Bring to a boil, reduce heat to low and simmer 10 minutes.
Transfer soup in batches to a blender and puree until smooth. Return to the pot and stir in feta cheese. Continue cooking 10 minutes.
Stir the prawns into the soup and continue cooking 3 minutes or until prawns are opaque. Mix in parsley just before serving.