Melt butter in a large skillet over medium-low heat. Add sliced onions, 1/2 tsp salt, 1/4 tsp black pepper, and granulated sugar. Cook, stirring occasionally, for 15-20 minutes, or until deeply golden brown and very soft.

Deglaze with dry sherry or beef broth, scraping up any browned bits from the bottom of the pan. Remove from heat and let cool slightly. Set aside about 1/4 cup of caramelized onions for the glaze.

Preheat oven to 375°F (190°C). Line a baking sheet with parchment paper or foil, and place a wire rack on top (this helps fat drain and prevents a soggy bottom).

In a large mixing bowl, combine ground beef, panko breadcrumbs, egg, 1/2 cup beef broth, minced garlic, 1 tsp fresh thyme, 1 tsp Worcestershire sauce, 1 tsp Dijon mustard, 1/2 tsp salt, and 1/4 tsp black pepper. Add the majority of the cooled caramelized onions (everything except the 1/4 cup reserved for the glaze).

Gently mix until just combined. Be careful not to overmix, as this can make the meatloaf tough. Form the mixture into a loaf shape (about 9x5 inches) on the prepared wire rack.

Bake for 50-60 minutes, or until the internal temperature reaches 150°F (65°C).

While the meatloaf bakes, in a small saucepan, combine 1/2 cup beef broth, 1 tbsp Worcestershire sauce, 1 tsp Dijon mustard, 1/2 tsp fresh thyme, and the reserved 1/4 cup of caramelized onions. Bring to a simmer over medium heat.

In a small bowl, whisk together the cornstarch and cold water to create a slurry. Whisk the slurry into the simmering glaze and cook, stirring constantly, until the glaze thickens slightly, about 1-2 minutes. Remove from heat.

After the initial 50-60 minutes of baking, remove the meatloaf from the oven. Spread the prepared glaze evenly over the top of the meatloaf.

Return to the oven and continue baking for another 10-15 minutes, or until the internal temperature reaches 160°F (71°C) and the glaze is slightly set.

Remove the meatloaf from the oven and preheat your broiler. Sprinkle generously with shredded Gruyère cheese.

Broil for 2-3 minutes, watching carefully to prevent burning, until the cheese is melted, bubbly, and lightly golden brown.

Let the French Onion Meatloaf rest for 10-15 minutes before slicing and serving. This allows the juices to redistribute for a more tender and flavorful result.