Melt butter in a large skillet over medium heat. Add onions and 1/4 teaspoon salt and cook for 30 to 40 minutes until deeply golden, stirring often and adding reserved water a tablespoon at a time if browning threatens to burn.
Add wine and simmer for 2 minutes, scraping the skillet. Transfer onions to a 6-quart slow cooker and add broth and thyme. Season chops with remaining salt and pepper and arrange over the onions.
Cover and cook on low for 6 hours, checking after 5 hours, until fork-tender and at least 145°F, usually near 190°F for marbled blade chops. Lift pork carefully to a platter.
Transfer onion liquid to a saucepan, skim excess fat and bring to a simmer. Stir cornstarch mixed with cold water into the sauce for 2 minutes until lightly thickened.
Return chops to the warm empty crock, spoon some onion gravy over them and scatter Gruyere evenly across the meat. Cover and heat on high for 5 to 10 minutes until the cheese melts, then serve with remaining onion gravy.