In a skillet over medium heat, melt the butter. Add the sliced onions and a pinch of salt, stirring to coat. Cook slowly, stirring every few minutes, for 15–20 minutes until the onions turn golden brown and soft. For added depth, stir in balsamic vinegar during the last minute of cooking. Remove from heat and set aside.

Set your oven to 375°F (190°C). Lightly grease a loaf pan or line it with parchment paper for easy cleanup.

In a large mixing bowl, combine ground beef, egg, shredded mozzarella, Parmesan cheese, finely chopped raw onion, minced garlic, Worcestershire sauce, salt, pepper, and dried thyme. Use your hands or a wooden spoon to mix gently until just combined—overmixing can make the meatloaf dense.

Press half of the meat mixture into the prepared loaf pan, forming an even layer. Spread half of the caramelized onions over the top, then sprinkle with half of the shredded cheese.

Top with the remaining meat mixture, pressing lightly to seal the edges. Smooth the surface, then bake uncovered for 45 minutes.

Remove from the oven and top with the remaining caramelized onions and shredded cheese.

Return the meatloaf to the oven for another 10–15 minutes, or until the internal temperature reaches 160°F (71°C) and the cheese is melted and bubbly.

Let the meatloaf rest for at least 10 minutes before slicing. Serve hot with mashed potatoes, sautéed greens, or crusty bread to soak up the juices.