In a large pot, heat olive oil over medium heat. Add the prawns, clams, mussels, fresh fish, red pepper, onion, tomato, garlic, chili flakes, basil, oregano, rosemary, and parsley and cook for 2 minutes. Add the lobster stock and then stir in the tomato puree. Cook until the clams and mussels open and sauce thickens, about 15 to 20 minutes. Season with salt and freshly ground black pepper. To serve, ladle into 2 bowls and garnish with crab.