Preheat your oven to 375 degrees. In a large mixing bowl, combine the ground beef, eggs, panko, Dijon mustard, beef broth, Worcestershire, thyme, salt, pepper, caramelized onions, and shredded Gruyère. Mix gently with your hands or a spatula. Avoid overmixing to keep the loaf tender. Shape into a loaf on a parchment-lined baking sheet or in a lightly oiled loaf pan. Bake for 45–50 minutes, or until the internal temperature reaches 160°F. During the last 10 minutes, spoon some of the onion gravy over the top and sprinkle with the remaining ¼ cup cheese. Broil for 1–2 minutes at the end until the cheese is golden and bubbly.

While the loaf bakes, finish the gravy right in the same pan with your remaining caramelized onions. Sprinkle in the flour and stir for 1 minute. Slowly whisk in the beef broth and Worcestershire. Simmer for 3–5 minutes, stirring often, until thickened. Taste and adjust seasoning as needed.

Let the meatloaf rest for about 10 minutes before slicing. Serve topped with a generous spoonful of onion gravy and a sprinkle of fresh thyme.