Preheat your oven to 350°F (175°C).
In a skillet, heat the olive oil over medium heat. Add the sliced onions and sugar; cook, stirring occasionally, until the onions are caramelized and golden brown, about 15-20 minutes. Remove from heat and let cool slightly.
In a large mixing bowl, combine the ground beef, ground pork, breadcrumbs, eggs, milk, Worcestershire sauce, salt, pepper, garlic powder, and dried thyme. Mix until just combined.
Fold in the cooled caramelized onions into the meat mixture until evenly distributed.
Transfer the meat mixture into a baking dish or loaf pan, shaping it into a loaf.
Cover the loaf with aluminum foil and bake for 40 minutes in the preheated oven.
After 40 minutes, remove the foil and lay the slices of Swiss cheese on top of the meatloaf. Return to the oven and bake for an additional 10-15 minutes, or until the cheese is melted and bubbly, and the meatloaf reaches an internal temperature of 160°F (70°C).
Let the meatloaf rest for about 10 minutes before slicing and serving.