Preheat the oven to 425 degrees F. Put the mirliton halves cut-side down in a 9-by-13-inch baking dish. Add 1 cup of water, cover tightly with foil and bake until soft, cooked through and easily poked with a fork, about 1 hour 15 minutes. Set aside until cool enough to handle, about 20 minutes. (Leave the oven on.) Scoop out the pulp--making sure to leave a 1/4-inch border all around--then chop the pulp and reserve. Discard the water from the baking dish and return the scooped-out mirliton halves cut-side down.
Meanwhile, melt 6 tablespoons of the butter with the Cajun seasoning in a large skillet over medium-high heat. Add the celery, garlic, bell pepper, onion and 1 teaspoon salt and cook, stirring occasionally, until the vegetables are soft and just begin to brown, 7 to 8 minutes. Add the sausage, hot sauce and tomatoes and cook, stirring, until the sausage is warmed through and the tomatoes start to release their juices, about 3 minutes. Add the crab, shrimp and reserved mirliton pulp and continue cooking until heated through, 2 to 3 minutes.
Microwave the remaining 2 tablespoons butter in a small microwave-safe dish until melted, about 30 seconds. Add the breadcrumbs and parsley and stir until well combined.
Turn each mirliton half over so the cut side is facing up. Stuff each with about 1/2 cup of the crab and shrimp mixture--it will seem like a lot, but that's OK. Sprinkle with the breadcrumb mixture and bake until the shrimp are cooked through and the breadcrumbs turn a deep golden brown, 10 to 12 minutes. Serve with additional hot sauce if desired.
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