Season the pork chops with salt and pepper on both sides. Heat the olive oil in a large skillet over medium-high heat and brown the chops for 2 to 3 minutes per side. Transfer them to the slow cooker.
In the same skillet, melt the butter over medium heat. Add the sliced onions and cook, stirring often, for 8 to 10 minutes until they soften and start to turn golden.
Stir in the garlic and thyme and cook for 1 minute more. Pour in the beef broth and scrape the bottom of the skillet to loosen any browned bits.
Transfer the onion mixture to the slow cooker, arranging it around and over the pork chops. Cover and cook on low for 6 hours.
Remove the pork chops to a plate. Stir the heavy cream and 1 cup of the Gruyère cheese into the onion sauce until the cheese melts and the sauce thickens slightly.
Return the pork chops to the slow cooker and sprinkle the remaining 1/2 cup Gruyère over the top. Cover and cook on low for 15 minutes more until the cheese melts.