### Instructions
1. **Caramelize the Onions (about 30-40 minutes):**
* In a large skillet over medium-low heat, melt the butter. Add the thinly sliced onions, salt, and pepper.
* Cook slowly, stirring occasionally, until the onions are deeply golden brown and very soft. This process takes time, but it’s worth it for the incredible flavor! Ensure they don’t burn. Remove from heat and let cool slightly.
2. **Prepare the Meatloaf Mixture:**
* In a large bowl, combine the ground beef, breadcrumbs, milk, egg, Worcestershire sauce, dried thyme, garlic powder, salt, and black pepper.
* Gently mix until just combined. Be careful not to overmix, as this can make the meatloaf tough.
3. **Assemble the Stuffed Meatloaf:**
* Lay a large piece of parchment paper or plastic wrap on your work surface.
* Spread the meatloaf mixture evenly into a rectangle, about 10×12 inches and ½ inch thick.
* Spoon the cooled caramelized onions over the meatloaf rectangle, leaving a 1-inch border on all sides.
* Evenly sprinkle the shredded Swiss cheese over the caramelized onions.
* Starting from one of the long ends, carefully roll up the meatloaf tightly, using the parchment paper to help you. Pinch the seams and ends to seal.
* Carefully transfer the rolled meatloaf to a baking dish or a loaf pan.
4. **Prepare the Glaze:**
* In a small bowl, whisk together the ketchup, brown sugar, apple cider vinegar, and Dijon mustard.
5. **Bake the Meatloaf:**
* Preheat your oven to 350°F (175°C).
* Bake the meatloaf for 45 minutes.
* Remove from the oven, spread the glaze evenly over the top of the meatloaf.
* Return to the oven and bake for another 15-20 minutes, or until the internal temperature reaches 160°F (71°C).
6. **Rest and Serve:**
* Once baked, remove the meatloaf from the oven and let it rest for 10-15 minutes before slicing. This allows the juices to redistribute, ensuring a tender and moist meatloaf.
* Slice thickly and serve with your favorite side dishes!
### Short Tips
* **Don’t Rush the Onions:** Caramelizing onions takes time, but don’t try to speed it up with high heat. Low and slow is the key to deep, sweet flavor without burning.
* **Prevent a Crumbly Loaf:** Only mix the meatloaf ingredients until just combined. Overmixing develops the proteins and can lead to a tough, dry, and crumbly meatloaf.
* **The Power of Parchment:** Using parchment paper makes rolling the meatloaf surprisingly easy and prevents sticking.
* **Rest is Best:** Always rest your meatloaf (and any roasted meat!) before slicing. This traps the juices inside, resulting in a more flavorful and tender slice.
* **Make it Ahead:** You can prepare the caramelized onions a day or two in advance. Store them in the refrigerator until ready to use. You can also assemble the meatloaf and store it in the fridge for up to 24 hours before baking.
* **Customization:** Feel free to experiment with other cheeses like Gruyere or white cheddar. You can also add a pinch of red pepper flakes to the onions for a subtle kick.
* **Serving Suggestions:** This meatloaf pairs wonderfully with mashed potatoes, roasted vegetables, green beans, or a simple side salad.
