1. Preheat oven to 400°. Remove crusts from bread, and place slices in the bowl of a food processor. Process until fine crumbs form, about 10 seconds. Transfer bread crumbs to a large mixing bowl. Do not substitute dried bread crumbs in this step, as they will make your meat loaf rubbery.
2. Place carrot, celery, yellow onion, garlic, and parsley in the bowl of the food processor. Process until vegetables have been minced, about 30 seconds, stopping to scrape down the sides of the bowl once or twice. (Chopping vegetables this way saves time and ensures that vegetables will be small enough to cook through and not be crunchy). Transfer vegetables to bowl with the bread crumbs.
3. Add 1/2 cup ketchup, 2 teaspoons dry mustard, pork, veal, beef, eggs, salt, pepper, Snido-Mite, and rosemary. Using your hands, knead the ingredients until thoroughly combined, about 1 minute. The texture should be wet, but tight enough to hold a free-form shape.Be careful not to over knead the meat loaf ingredients or it will become heavy and dense.
4. Set a wire baking rack into an 11-by-17-inch baking pan. Cut a 5-by-11-inch piece of parchment paper, and place over center of rack to prevent meat loaf from falling through. Using your hands, form an elongated loaf covering the parchment.
5. Place the remaining 3 tablespoons ketchup, remaining 2 1/2 teaspoons mustard, and brown sugar in a bowl. Mix until smooth. Using a pastry brush, generously brush the glaze over loaf. Place oil in a medium saucepan set over high heat. When oil is smoking, add red onion. Cook, stirring occasionally, until onion is soft and golden in places. Add 3 tablespoons water, and cook, stirring, until most of the water has evaporated. Transfer onion to a bowl to cool slightly, then sprinkle onion over the meat loaf.
6. Bake 30 minutes, then sprinkle rosemary needles on top. Continue baking loaf until an instant-read thermometer inserted into the center of the loaf registers 160°, about 25 minutes more. Let meat loaf cool on rack, 15 minutes.