I start by slowly cooking the sliced onions in butter over medium-low heat until golden brown and sweet. This process takes about 15–20 minutes, but it’s worth the wait. Don’t rush it—slow caramelization brings out the rich flavor.
In a large bowl, I combine the ground beef, bread crumbs, milk, egg, Worcestershire sauce, garlic, thyme, salt, and pepper. I gently mix everything with my hands, just enough to combine. Overmixing can make the meatloaf tough.
I spread half of the meat mixture into a loaf pan, layer the caramelized onions on top, then cover with the remaining meat mixture. Finally, I sprinkle shredded Swiss cheese evenly over the top for a melty, golden crust.
I bake the meatloaf at 350°F (175°C) for 55–60 minutes. The internal temperature should reach 160°F for perfectly cooked beef. I let it rest for 10 minutes before slicing to lock in juices.