Caramelize Onions: Heat butter in a skillet over medium heat. Add onions and cook for 20-25 minutes until golden and caramelized. Set aside to cool.
Preheat Oven: Preheat oven to 375°F (190°C).
Combine Ingredients: In a bowl, mix ground beef, half the caramelized onions, seasoning mix, breadcrumbs, egg, and half the cheese. Season lightly.
Form and Bake: Shape mixture into a loaf and place on a baking sheet. Bake for 40-45 minutes.
Top with Cheese: Add remaining onions and cheese on top, then bake for 5-7 more minutes.
Rest and Serve: Let rest 10 minutes before slicing. Garnish and enjoy!