Preheat the oven to 350 degrees.
For six people as a main dish, allow at least one whole mirliton per person, or half a mirliton per person if you are using it as a side dish.
Choose plump firm mirliton. In a heavy pot, bring about 2 inches of salted water to a boil. Add the mirliton, cover, and reduce heat and simmer until they are tender, about 20 minutes. Remove from pot and allow to cool. Then cut the mirliton in half lengthwise, and remove the seeds. Scoop out the flesh keeping the shell intact. Chop the flesh and set aside.
Heat a tablespoon olive oil in a large skillet over medium heat. Add the chopped vegetables, the chopped flesh, the bay leaf, and thyme. Cook, stirring occasionally, until the any liquid in the skillet evaporates, 5 to 8 minutes. Season with salt and peppers. When the vegetables are very tender, increase the heat to medium-high, add the shrimp, and cook until the shrimp are pink and firm. Remove from the heat and remove the bay leaf. Add the crabmeat and stir gently to mix. Adjust seasoning. Mirliton has a delicate flavor, greatly enhanced by shrimp. This is not meant to be a fiery hot dish, simply a savory experience, so don't over season.
Spoon the mixture into the mirliton shells, dust the top with the breadcrumbs, and bake until the breadcrumbs are lightly browned, about 30 minutes.
These general directions work well for eggplant or any of the squashes as well. Adjust the boiling time for the vegetables according to the size of the vegetable. Zucchini may take only about 5 minutes. Be sure to salt the eggplant after it has been scooped out of the shell, and let drain for an hour before cooking. The salt will draw out any bitterness in the eggplant. After an hour, rinse, pat dry and continue. Remember, as you are seasoning, that the eggplant is already salty. I use more pepper and spices in eggplant than I do in mirliton or squash dishes.