- Shape and Bake: Transfer the mixture onto a parchment-lined baking sheet and shape it into a loaf. Bake uncovered for 35-40 minutes.
- Prepare the Cheese Sauce: While the meatloaf bakes, melt the butter in a small saucepan over medium heat. Stir in the flour to create a roux and cook for 1-2 minutes. Gradually whisk in the milk, followed by the remaining beef broth, and continue to cook until the sauce has thickened. Remove from the heat and stir in the shredded mozzarella and Gruyère until the sauce is smooth and velvety.
- Assemble and Broil: Remove the meatloaf from the oven. Top it with the remaining caramelized onions and generous spoonfuls of the cheese sauce. Sprinkle with a little extra shredded Gruyère for a bubbly crust. Return the meatloaf to the oven and place it under the broiler for 3-5 minutes, or until the topping is golden brown and lightly crisped.
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