Slice 2 large onions thinly. Melt 3 tablespoons of butter in a skillet over medium-low heat. Add onions and cook for 20-25 minutes until caramelized.

Sprinkle 2 tablespoons of flour over caramelized onions and whisk to form a roux. Gradually stir in 1 cup of beef broth until thickened.

In a mixing bowl, combine 1 pound ground beef, 1/2 pound ground pork, 1 beaten egg, 1 cup breadcrumbs, and 1 cup shredded Gruyère cheese. Add 1 teaspoon thyme, salt, and pepper to taste, and mix gently.

Fold half of the caramelized onions into the meat mixture, ensuring even distribution without overmixing.

Transfer meat mixture to a greased loaf pan, shaping it evenly. Top with remaining Gruyère cheese. Bake at 350°F for 50-60 minutes until the internal temperature reaches 160°F.

Let the meatloaf rest for 10 minutes, then serve each slice with reserved caramelized onion gravy.