1. Caramelize the Onions
Heat olive oil in a large skillet over medium heat. Add sliced onions and a pinch of salt. Cook, stirring frequently, for about 25–30 minutes until they’re deeply golden and sweet. Occasionally add a splash of beef broth to deglaze the pan and deepen the flavor. Stir in the minced garlic and cook for 2 more minutes.
Pro tip: Slow and steady caramelization is key for rich flavor.
2. Mix the Meatloaf
In a large bowl, combine ground beef, panko breadcrumbs, eggs, Worcestershire sauce, Dijon mustard, fresh thyme, salt, pepper, and half of the caramelized onions. Mix gently until just combined — overmixing will make the meatloaf dense.
3. Shape and Bake
Preheat oven to 375°F (190°C). Shape the mixture into a loaf on a parchment-lined baking sheet or place in a loaf pan. Bake uncovered for 35–40 minutes.
4. Make the Cheese Sauce
While the meatloaf bakes, melt butter in a saucepan. Stir in flour and cook for 1–2 minutes to form a roux. Gradually whisk in milk, then the remaining beef broth, cooking until thickened. Remove from heat and stir in shredded mozzarella and Gruyère until smooth.
5. Assemble and Broil
Take the meatloaf out of the oven. Spread the remaining caramelized onions on top, spoon over the cheese sauce, and sprinkle extra shredded Gruyère. Broil for 3–5 minutes until the topping is golden and bubbly.