Boil mirlitons in salted water until tender (about 45 minutes). Remove from heat and drain. Cut mirlitons in half. Remove the seeds and carefully spoon out the pulp.
Chop up the pulp and add 1 cup Italian breadcrumbs. Saute onions, garlic, parsley and shrimp in butter over medium heat until tender, about 10 to 15 minutes. Mix in pulp mixture, salt and pepper; continue cooking for 5 minutes, stirring frequently. Taste for seasoning and add salt and pepper if needed.
Fill shallow casserole dish with mixture and sprinkle top with ¼ cup breadcrumbs. Add pats of butter to top. Bake in 350 degree oven for 25 minutes or until lighted browned.