Preheat your oven to 350°F (175°C) and lightly grease a medium-sized baking dish.

In a skillet over medium heat, melt 2 tablespoons of butter and add sliced onions. Cook for 20-25 minutes until caramelized, then deglaze the pan with beef broth.

In a large bowl, combine ground beef, breadcrumbs, and milk. Add minced garlic, egg, Worcestershire sauce, Dijon mustard, thyme, salt, and pepper. Mix gently.

Fold in cooled caramelized onions and grated Gruyère cheese until evenly distributed.

Shape the meat mixture into a loaf in the greased baking dish.

Bake in the preheated oven for 1 hour until internal temperature reaches 160°F (70°C).

Sprinkle remaining 1/2 cup of Gruyère cheese on top and bake for an additional 5-10 minutes until melted.

Allow meatloaf to rest for at least 10 minutes before slicing and serving.