Soak the salt pork overnight in cold water before cooking.
To cook, rinse it in cold water, place in a saucepan, cover with cold water, bring to a boil, then lower the heat and simmer for about an hour. Drain and leave to cool before using to stuff the tomatoes.
Wash the tomatoes, then use a small knife to cut off the top third of each one. Reserve the tops. Use a spoon to carefully scoop out the seeds and pulp from the center of each tomato, being careful not to puncture the skins. Save the scooped-out pulp and seeds for making the sauce. Lightly salt the inside of each tomato and reserve.
Preheat the oven to 400°F.
Place the seeds and pulp in a small saucepan with a little thyme, bay leaf, salt and pepper, and simmer for 15-20 minutes or until the consistency of a thin tomato sauce, stirring frequently. Reserve.
While the sauce is cooking, place the onion in a saucepan with the milk, bring to a boil, then lower the heat and simmer 7-8 minutes. Lift the onions out of the milk with a slotted spoon and chop them coarsely. Save the milk.
Place the toast in a mixing bowl and add the milk used in cooking the onions. Allow to soak for about 10 minutes, then crush the toast with a fork; pour off any excess milk.
Chop the salt pork, and add it to the toast along with the onion, garlic, leftover meat and parsley. Mix well, then add the egg, salt, pepper, and nutmeg, and knead with your hands to mix thoroughly.
Butter a baking dish just large enough to hold all the tomatoes. Fill each tomato with stuffing, then put the top sections back in place and arrange them in the baking dish around the tomatoes, then place in the over and bake for 1 hour. Serve hot from the oven in the baking dish.