01 -
Heat olive oil in a large skillet over medium heat. Add thinly sliced onions with a pinch of salt and cook, stirring frequently, allowing them to soften and caramelize for 25-30 minutes. Occasionally deglaze the pan with a splash of beef broth. Once deeply golden, stir in minced garlic and cook for 2 additional minutes. Reserve half the caramelized onions for topping.
02 -
Preheat oven to 375°F (190°C). 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, avoiding overmixing to retain tenderness.
03 -
Transfer mixture onto a parchment-lined baking sheet and form into a loaf shape. Bake uncovered for 35-40 minutes until cooked through.
04 -
Melt butter in a small saucepan over medium heat. Stir in flour to form a roux and cook for 1-2 minutes. Gradually whisk in milk followed by beef broth, cooking until sauce thickens. Remove from heat and stir in shredded Mozzarella and Gruyère until smooth and creamy.
05 -
Remove meatloaf from oven. Top with remaining caramelized onions and spoon over cheese sauce. Sprinkle extra shredded Gruyère for a golden crust. Return to oven under broiler for 3-5 minutes until topping is bubbly and lightly browned.