Combine the softened unsalted butter, grated garlic cloves, chopped parsley, and kosher salt for garlic butter in a small bowl. Mix until smooth and set aside.

Combine the ground beef, breadcrumbs, grated Gruyère cheese, chopped parsley, eggs, kosher salt, and black pepper in a large mixing bowl. Mix only until the ingredients are evenly distributed.

Divide the meat mixture into 16 equal portions. Roll each portion into a meatball.

Heat the cast-iron skillet over medium-high heat. Sear the meatballs for 6–8 minutes, turning with tongs until deeply browned on all sides.

Transfer the browned meatballs from the skillet to a clean plate. Keep them aside; they do not need to be cooked through yet.

Reduce the stovetop heat to medium and add the neutral oil to the same skillet. Add the sliced yellow onions.

Cook the onions for 30–40 minutes. Stir occasionally until they are deeply caramelized and soft.

Stir the Worcestershire sauce, fresh thyme leaves, and remaining black pepper into the caramelized onions. Cook for 1 minute.

Add the beef demi-glace and beef stock to the skillet. Scrape the browned bits from the bottom of the skillet.

Bring the French onion sauce to a gentle simmer. Return all 16 meatballs to the skillet, nestling them into the onions.

Spread the shredded Gruyère cheese evenly over the meatballs and onions. Transfer the skillet to a 400°F / 205°C oven for 10–15 minutes.

Check that the meatballs reach 160°F / 71°C internally. Remove the skillet when the cheese is melted and bubbling.

Slice each crusty baguette lengthwise in half. Spread the cut sides generously with the prepared garlic butter.

Toast the buttered baguettes until golden and crisp around the edges. Remove them from the oven.

Place four meatballs into each toasted baguette. Spoon caramelized onions, melted Gruyère, and French onion jus over each sub.