Heat the olive oil in a large skillet over medium-low heat. Add the sliced onions and ½ teaspoon of the salt. Cook, stirring often, for 25 to 30 minutes until the onions are soft and deeply golden. If the skillet gets dry, add a spoonful of the listed beef broth. Set the onions aside to cool.

Preheat an electric smoker to 250°F. Use the smoker’s water pan and wood chips according to the manufacturer’s instructions.

In a mixing bowl, combine the ground beef, egg, breadcrumbs, milk, Worcestershire sauce, Dijon mustard, ½ teaspoon of the thyme, black pepper, and the remaining ½ teaspoon of salt. Mix gently until just combined.

Fold in half of the cooled caramelized onions and ½ cup of the Gruyère. Shape the mixture into a compact loaf and place it on a smoker-safe tray.

Stir the remaining onions into the beef broth and apple cider vinegar. Spoon this mixture over the top of the loaf, then place the tray in the smoker. Smoke until the center reaches 160°F on an instant-read thermometer, checking after about 2 hours.

Sprinkle the remaining Gruyère and thyme over the meatloaf. Smoke for another 5 to 10 minutes, until the cheese melts. Rest the loaf for 10 minutes before slicing so it holds together.