1. In a large skillet over medium-low heat, melt butter and add sliced onions with a pinch of salt. Cook slowly, stirring occasionally, until deeply caramelized (about 30–40 minutes). Deglaze with a splash of beef stock. Set aside.

2. In a mixing bowl, combine ground beef, breadcrumbs, egg, Worcestershire sauce, garlic, salt, pepper, fresh thyme, and half of the caramelized onions. Mix until just combined.

3. Shape the mixture into a loaf on a parchment-lined baking sheet or place in a loaf pan.

4. Bake in a preheated oven at 375°F (190°C) for 40 minutes.

5. Remove from oven. Top with Swiss cheese slices and the remaining caramelized onions.

6. Return to oven and bake for an additional 10–15 minutes, or until the cheese is melted and internal temp reaches 160°F (71°C).

7. Let rest for 5–10 minutes before slicing. Serve warm and garnish with fresh thyme if desired.