Preheat a smoker to 250°F. Set up a drip pan beneath the cooking grate. If your smoker requires a specific type of wood, use your preferred mild wood for a balanced smoky flavor.

Heat the olive oil in a skillet over medium heat. Add the finely chopped mushrooms and the 1/4 cup chopped onion. Cook for 5 to 7 minutes, stirring occasionally, until softened and most of the moisture has evaporated. Set aside to cool.

In a mixing bowl, combine the ground beef, egg, breadcrumbs, cooled mushroom mixture, Swiss cheese, Worcestershire sauce, salt, and black pepper. Mix just until combined, then shape into a compact loaf.

Lay the bacon slices side by side on a clean surface, slightly overlapping them. Place the meatloaf in the center and wrap the bacon around it, tucking the ends underneath. Set the loaf seam-side down in a smoker-safe pan.

Smoke the meatloaf until its center reaches 160°F on an instant-read thermometer, about 2 to 3 hours depending on the smoker and loaf shape. Let it rest for 10 minutes before slicing.

While the meatloaf smokes, make the onion gravy. Add the sliced onion to a skillet over medium heat and cook, stirring occasionally, until soft and golden, about 12 to 15 minutes.

Sprinkle the flour over the onions and stir for 1 minute. Gradually pour in the beef broth while stirring to keep the gravy smooth. Stir in the Dijon mustard and simmer until thickened, about 3 to 5 minutes.

Slice the rested meatloaf and spoon the onion gravy over the top. Finish with the chopped parsley.