1) Preheat the oven to 180C/Gas 4. Bring a large saucepan of water to the boil over a high heat and add about 1/2 teaspoon of Cajun seasoning. Add the mirlitons, lower the heat and cook for 30 to 45 minutes until they are the consistency of a soft potato.
2) Drain the mirlitons. Put a knife in each crease and cut the mirlitons in half lengthwise. Scoop out the pulp, leaving at least 1/2cm all around. Set the pulp aside. Turn the mirlitons upside down to let them drain.
3) In a frying pan over a medium heat, melt the butter and add the chopped vegetables, cooking until soft, about 5 to 10 minutes. Chop the mirliton pulp and add to the pan. Add the remaining 1/2 teaspoon of Cajun seasoning and the prawns and stir to combine. Stir in the breadcrumbs and season with salt and pepper, to taste.
4) Place the mirliton halves into a casserole dish. Scoop the prawn mixture into the mirliton shells and bake for 30 minutes. Serve hot.