Step 1: In a skillet, melt 2 tablespoons of unsalted butter over medium-low heat. Add 3 sliced yellow onions, a pinch of salt, black pepper, and 1 teaspoon of granulated sugar. Cook the onions gently for 20-25 minutes, stirring occasionally, until they become golden brown and soft.

Step 2: Once the onions are caramelized, add ¼ cup of dry sherry or beef broth to the skillet. Stir to incorporate the flavors and set half aside.

Step 3: Preheat the oven to 375°F (190°C). In a mixing bowl, combine 1 pound of ground beef, ½ cup of soaked panko breadcrumbs, 1 large egg, 3 minced garlic cloves, 1 tablespoon of fresh thyme leaves, 1 tablespoon Worcestershire sauce, and 1 tablespoon Dijon mustard. Incorporate the majority of the caramelized onions.

Step 4: Shape the meat mixture into a loaf on a wire rack over a baking sheet. Bake for 50-60 minutes until the internal temperature reaches 150°F (65°C).

Step 5: For the glaze, simmer the reserved onion mixture in a saucepan until it thickens.

Step 6: Brush the glaze on the meatloaf and return it to the oven to bake for an additional 10-15 minutes until it reaches 160°F (71°C).

Step 7: Remove the meatloaf and sprinkle 1 cup of shredded Gruyère cheese on top. Broil for 2-3 minutes until melted and bubbly.

Step 8: Let the meatloaf rest for 10 minutes before slicing. Serve warm.