Heat the olive oil and butter in a frying pan over medium heat. Stir in the capsicum and onion; cook and stir until the onion has softened and turned translucent, about 5 minutes. Stir in the garlic and cook until fragrant, about 1 minute more. Add the prawns and cook until opaque, about 2 minutes on each side. Stir in the tomato puree, parsley and chilli flakes then cook until sauce is warmed through, about 5 minutes. Season with salt and pepper.