Preheat the oven to 350?F.

Grease a 2-quart rectangular baking pan with the butter.

In a large saucepan, combine the mirlitons, water, bay leaves, 1 tablespoon of the salt, the cayenne, and Zatarain's boil, if using. Bring to a boil over medium-high heat and cook, uncovered, until the mirlitons are fork-tender, 40 to 50 minutes.

Remove from the heat and drain. Let cool for 30 minutes and peel. Cut the mirlitons in half lengthwise and remove the seeds. Cut each half into 1/2-inch-thick slices.

In a large sauté pan over medium heat, brown the sausage about 4 minutes. Transfer to a small bowl.

To the same sauté pan, add 2 tablespoons of the olive oil. Add the onions, the remaining 1/2 teaspoon salt, and the black pepper. Cook, stirring, until the onions are soft and lightly golden, about 5 minutes. Add the mirlitons, garlic, parsley, and green onions; cook, stirring, for 2 minutes, and remove from the heat. Pour the mixture into the prepared pan.

Combine the sausage, bread crumbs, cheese, Creole seasoning, and the remaining 2 tablespoons olive oil. Mix well. Sprinkle the mixture over the mirliton mixture, covering the top completely. Bake until the top is golden brown, about 30 minutes. Serve warm.