### Instructions:
1. **Caramelize the Onions:** Heat olive oil in a large skillet over medium-low heat. Add the sliced onions, thyme, 1/2 teaspoon salt, and 1/4 teaspoon pepper. Cook, stirring occasionally, for 25-30 minutes, or until the onions are deeply golden brown and caramelized. Remove from heat and let cool slightly.
2. **Prepare the Meatloaf Mixture:** In a large bowl, combine the ground beef, panko breadcrumbs, milk, egg, Worcestershire sauce, Dijon mustard, remaining 1/2 teaspoon salt, and 1/4 teaspoon pepper.
3. **Add Onions and Cheese:** Gently fold in the cooled caramelized onions and 1/2 cup of the shredded Swiss cheese. Be careful not to overmix, as this can make the meatloaf tough.
4. **Form the Loaf:** Lightly grease a loaf pan or line a baking sheet with parchment paper. Shape the meatloaf mixture into a loaf in the prepared pan or on the baking sheet.
5. **Make the Glaze:** In a small bowl, whisk together the beef broth and ketchup.
6. **Bake:** Preheat your oven to 375°F (190°C). Pour half of the glaze over the meatloaf. Bake for 45 minutes.
7. **Final Bake and Cheese:** Remove the meatloaf from the oven. Top with the remaining 1/2 cup of Swiss cheese and pour the remaining glaze over the cheese. Return to the oven and bake for another 15-20 minutes, or until the internal temperature reaches 160°F (71°C) and the cheese is melted and bubbly.
8. **Rest and Serve:** Remove the meatloaf from the oven and let it rest for 10-15 minutes before slicing and serving. This allows the juices to redistribute, resulting in a more tender and flavorful meatloaf.
### Short Tips:
* **Don’t Rush the Onions:** The key to French onion flavor is truly caramelized onions. Low and slow is the way to go!
* **Avoid Overmixing:** Overworking the meat can lead to a dry, tough meatloaf. Mix just until combined.
* **Customize Your Cheese:** While Swiss is classic for French onion, feel free to experiment with Gruyere or even provolone for a slightly different twist.
* **Make it Ahead:** You can prepare the meatloaf mixture up to a day in advance and store it in the refrigerator. Just bring it to room temperature before baking.
* **Leftover Potential:** This meatloaf makes fantastic sandwiches the next day! Slice it thin and warm it up in a pan.
