Preheat the oven to 350 degrees. Place the meatloaf slices on a baking sheet and warm them for 8 to 10 minutes until heated through.
While the meatloaf warms, melt the butter in a large skillet over medium heat. Add the sliced onions and cook for 10 to 12 minutes, stirring often, until they turn soft and golden.
Sprinkle the flour over the onions and stir for 1 minute. Slowly pour in the beef broth while stirring to avoid lumps. Add the Worcestershire sauce, salt, and pepper. Simmer for 5 minutes until the gravy thickens slightly.
Toast the sourdough bread slices until golden and crisp.
Place two warmed meatloaf slices on each piece of toast. Spoon the onion gravy generously over the top and sprinkle with chopped parsley before serving.