Blanch and peel the tomatoes, then quarter, seed, and chop them, reserving their juices. Put the tomatoes, reserved juices, and onion in a saucepan and heat over medium high. As soon as the mixture begins to boil, reduce the heat to a bare simmer and cook until the onions begin to soften, about five minutes. Turn off the heat. Position a rack in the center of the oven and preheat oven to 375°F. Choose a deep two-quart casserole dish that will hold all the ingredients in three layers and grease it well with butter. Put about a half cup of crumbs in the bottom of the dish, followed by a third of the shrimp and then a third of the tomatoes. Dot with butter and season well with a healthy pinch of salt, pepper, and/or mace or nutmeg. Repeat two more layers with remaining crumbs, shrimp, tomatoes, and seasonings, reserving enough crumbs to cover the top. Dot with butter and bake until nicely browned, about 20 to 30 minutes.