Sauté the Onions: Heat olive oil in a large skillet over medium heat, add sliced onions and sauté for about 10-15 minutes.

Caramelize the Onions: Sprinkle sugar, salt, and pepper over onions. Reduce heat to low and cook for 45-60 minutes until golden-brown.

Prepare the Beef: In the same skillet, cook ground beef over medium-high heat, breaking it apart until browned for 6-8 minutes.

Season the Beef: Add garlic powder, dried thyme, and red pepper flakes to the beef and cook for 1 minute.

Create the Sauce: Pour in beef broth and red wine, scraping browned bits and let it simmer for 15-20 minutes until sauce thickens.

Add Flavor Boosts: Stir in Worcestershire sauce and balsamic vinegar, mixing well and returning caramelized onions back to skillet.

Combine Ingredients: Fold caramelized onions into the beef mixture, cooking for another 2-3 minutes to meld flavors.

Final Seasoning: Stir in Dijon mustard, taste and adjust the seasoning with salt and pepper if necessary.

Assemble the Sloppy Joes: Spoon beef mixture onto buns, topping each with Gruyere cheese.

Broil the Cheese: Preheat broiler and broil assembled buns for 2-3 minutes until cheese is melted and bubbling.

Serve and Garnish: Remove from oven, garnish with parsley if desired, and serve immediately while hot.