If you’re in need of a breakfast casserole for hungry guests or even a savory one for dinner, this pork strata is perfect. Everyone will be loving the combination of crunchy bread cubes, creamy cheese, and tender pork.
Combine water, sugar, mustard seeds, bay leaf, and 1/3 cup of the salt in a large bowl; whisk until salt and sugar dissolve. Add pork chops, submerging completely; cover and chill 1 1/2 hours. Remove pork from brine; discard brine. Pat pork chops dry, and sprinkle with pepper and remaining 1 1/2 teaspoons salt.
Heat oil in a large skillet over medium-high. Cook pork chops until browned, 3 to 4 minutes per side. Remove chops from skillet. Add onions; cook, stirring occasionally, until softened, about 6 minutes. Add apple cider. Cook until reduced slightly, about 2 minutes. Transfer onion mixture to a 6-quart slow cooker; stir in chicken broth and mustard. Place pork chops on top of onion mixture. Cover and cook on LOW until pork chops are tender, about 4 hours. When the meat is cool enough to handle, chop it and set aside.
Preheat the oven to 375˚F. Toss the French bread cubes with 3 tablespoons olive oil, 1 teaspoon kosher salt, and 1/2 teaspoon black pepper. Bake in the preheated oven until lightly toasted, about 10 minutes.
Heat 1 tablespoon olive oil in a large nonstick skillet over medium-low. Add the sweet onion and 1/2 teaspoon kosher salt. Cook until deeply browned, about 30 minutes. The baby spinach; cook, until wilted, about 1 1/2 minutes. Cool 15 minutes.
Whisk together eggs, milk, 2 teaspoon kosher salt, 1/2 teaspoon black pepper, and dry mustard in a large bowl. Add 10 oz. chopped pork, 3 oz. Swiss cheese, bread cubes, and onion mixture to egg mixture; stir to combine. Pour into a lightly greased 11- x 7-inch baking dish. Place on a rimmed baking sheet. Sprinkle with remaining 3 oz. Swiss cheese.
Bake at 375°F until golden brown and set, about 45 minutes, shielding with foil during last 10 minutes to prevent excess browning, if needed.