Preheat an oven to 350 degrees F. Place the bread on a baking tray, slide it into the oven and toast until the slices are lightly golden, about 20 minutes. Remove the baking tray from the oven, and leave the bread slices out to cool. Using your hands or a food processor, crumble the toasted bread, until bread crumbs form. In a medium bowl, toss together the bread crumbs, parsley, zest, and garlic. Pour in 1/4 cup olive oil, and season with salt and black pepper, to taste.
Rub enough extra olive oil to coat the lamb racks. Season the meat liberally with salt and black pepper, to taste. In a large, hot frying pan, place the lamb racks, fat-side down, and cook over medium heat for 2 minutes. Transfer the racks to a baking tray, spread their tops evenly with the mustard. Press the bread crumb mixture into the meat. Roast until the lamb is fully cooked and nicely pink, about 25 to 30 minutes. Remove the lamb racks from the oven. Leave them out to rest, loosely covered with foil, for 5 minutes before serving.