Slice two onions into thick rounds and set aside. Finely chop the remaining onion.
In a large bowl, combine the ground beef, chopped onion, breadcrumbs, egg, garlic, Worcestershire sauce, thyme, 1 teaspoon salt, and 1/2 teaspoon black pepper. Mix until evenly blended but do not overwork.
Shape the mixture into a compact loaf about 8 inches long. Brush lightly with olive oil.
Heat a grill to medium heat. Place the meatloaf on the grates and cook for 25 to 30 minutes, turning once, until it reaches 160°F inside. Move the loaf to a plate and tent with foil.
While the meatloaf rests, place the onion rounds on the grill. Cook until deeply charred on both sides, about 4 minutes per side. Transfer the charred onions to a skillet over medium heat.
Add the butter and beef broth to the skillet. Scrape up any browned bits and simmer for 5 minutes until the sauce thickens slightly. Season with salt and pepper to taste.
Slice the meatloaf and serve with the charred onion pan sauce spooned over the top.