Preheat the oven to 400 degrees Fahrenheit (200 degrees Celsius). Spray a baking sheet with nonstick cooking spray and set it aside.

In a skillet preheated to medium heat, melt the butter. Add the onion and cook for a few minutes, until softened. Season with salt, pepper, and sugar.

Add the Worcestershire sauce and thyme to the skillet and cook for another minute.

Pour in the beef broth and bring to a simmer. Cook for 10-15 minutes, or until the onions are golden brown. If the sauce starts to get too thick, you can add a little more beef broth.

In a large bowl, combine the ground beef with the cooked onions and broth. Mix gently until just combined.

Using an ice cream scooper or your hands, form the mixture into 9 meatballs. Place the meatballs on the prepared baking sheet.

Bake the meatballs for 12-14 minutes, or until cooked through.

While the meatballs are baking, prepare your hoagie sub rolls. You can toast them in the oven or on the grill if you like.

Once the meatballs are cooked, take a hoagie roll and stuff it with 3 meatballs.

Place the filled sub on a baking sheet and top with 1/4 cup of grated provolone cheese. Repeat with the remaining meatballs and cheese.

Turn your oven to broil. Broil the subs for 1-2 minutes, or until the cheese is melted and bubbly.

Serve the French Onion Meatball Subs immediately.