One 12-ounce loaf rustic white bread, cut into 1/2-inch dice (7 cups)
4 ounces thinly sliced smoked turkey, cut into 1/4-inch ribbons
6 ounces thinly sliced Swiss cheese, cut into 1/2-inch ribbons
Preheat the oven to 375°. Set a rack in the top third of the oven. Lightly grease a 8-by-11 1/2-inch baking dish. In a large skillet, heat the olive oil. Add the onion and cook over moderately high heat until browned, about 8 minutes
In a large bowl, whisk the milk with the eggs, mustard, salt, thyme and pepper. Fold in the bread, onion, turkey and two-thirds of the Swiss cheese. and the turkey. Let stand until most of the liquid has been absorbed, about 10 minutes.
Scrape the bread mixture into the prepared baking dish and sprinkle with the remaining Swiss cheese. Bake the bread pudding until golden and bubbling, about 45 minutes. Let stand for 5 minutes before serving.
The bread pudding can be baked up to one day in advance. Gently rewarm before serving.