Preheat your oven to 350°F (175°C).
Melt the butter in a large skillet over medium heat. Add the sliced onions and sugar and cook, stirring occasionally, until the onions are caramelized and golden brown, about 20 minutes.
In a large bowl, combine the ground beef, ground pork, breadcrumbs, eggs, milk, Worcestershire sauce, salt, pepper, and thyme. Mix until well blended.
Fold in the caramelized onions, reserving a small portion for the meatloaf topping later.
Transfer the meat mixture to a loaf pan and press down lightly to form an even loaf.
Top the meatloaf with the reserved caramelized onions.
Bake in the preheated oven for 45 minutes.
Remove from the oven and sprinkle with the shredded Swiss cheese.
Return to the oven and bake for another 15 minutes, or until the cheese is melted and bubbly.
Let the meatloaf rest for 10 minutes before slicing and serving.
Variations and Tips
For a different flavor profile, you can use Gruyère cheese instead of Swiss cheese for a nuttier taste. You can also add a tablespoon of Dijon mustard to the meat mixture for a tangy touch. If you prefer something spicier, add a pinch of cayenne pepper or some chopped jalapeño peppers. For a gluten-free version, replace the breadcrumbs with gluten-free breadcrumbs or crushed rice crackers.
