Mix the ground beef, egg, breadcrumbs, Worcestershire sauce, salt, and pepper in a bowl until just combined. Shape into a loaf that will fit inside your slow cooker.
Heat the butter and olive oil in a skillet over medium heat. Add the sliced onions and cook, stirring occasionally, until they turn golden and soft, about 15 minutes.
Stir most of the thyme into the onions, reserving a pinch for the gravy. Spread half the onion mixture across the bottom of the slow cooker. Place the meatloaf on top and cover it with the remaining onions.
Pour the beef broth around the meatloaf. Cover and cook on low for 6 hours.
Remove the meatloaf and let it rest. Stir the remaining thyme and a splash of the cooking liquid into the onions in the slow cooker to finish the gravy. Slice the meatloaf and serve with the onion gravy spooned over the top.