Thinly slice two onions and set them aside for the sauce. Finely chop the remaining onion.
In a mixing bowl, combine the ground beef, chopped onion, breadcrumbs, egg, Worcestershire sauce, minced garlic, thyme, and a pinch of salt and pepper. Mix until just blended, then shape into a loaf.
Soak the hickory wood chips for 30 minutes. Prepare a smoker to 275 degrees and add the chips. Place the meatloaf on the grate and smoke for about 90 minutes, or until the internal temperature reaches 160 degrees.
While the meatloaf smokes, melt the butter in a large skillet over medium heat. Add the sliced onions and cook slowly, stirring often, until they turn deep golden brown, about 25 minutes.
Pour in the bourbon and let it reduce for two minutes. Add the beef broth and simmer until the sauce thickens slightly. Season with salt and pepper.
Rest the meatloaf for 10 minutes, then slice and serve with the warm bourbon onion sauce spooned over the top.