Caramelize the Onions:
In a large skillet, melt butter with olive oil over medium heat.
Add sliced onions, salt, and sugar. Cook, stirring occasionally, until onions are golden brown and caramelized, about 20-25 minutes.
Stir in beef broth to deglaze the pan, scraping up any browned bits. Cook for an additional 2 minutes. Remove from heat and set aside.
Prepare the Meatloaf:
Preheat oven to 350°F (175°C). Line a baking sheet with parchment paper.
In a large bowl, combine ground beef, breadcrumbs, eggs, beef broth, Worcestershire sauce, salt, pepper, garlic powder, and dried thyme. Mix until just combined.
Fold in half of the caramelized onions and 1 cup of the shredded Gruyère cheese into the meat mixture.
Shape the mixture into a loaf on the prepared baking sheet.
Bake:
Bake in the preheated oven for 45 minutes.
Remove from oven, top with remaining caramelized onions and ½ cup shredded Gruyère cheese.
Return to oven and bake for an additional 15 minutes, or until the internal temperature reaches 160°F (71°C).
Prepare the Gravy (Optional):
In a small saucepan, whisk together beef broth, cornstarch, Worcestershire sauce, salt, and pepper.
Bring to a simmer over medium heat, stirring constantly until thickened.
Serve:
Let the meatloaf rest for 10 minutes before slicing.
Serve with gravy drizzled over the top, if desired.