How to Make It
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.