Melt the butter in a large pan on medium-low heat. Then add the onions. Cover and cook for 15-20 minutes until the onions start to sweat and soften.

Uncover the pan and stir in the Worcestershire sauce and beef bouillon paste. Cook uncovered on low heat, stirring regularly for another 20-30 minutes until the onions are caramelized.

In a large bowl, combine half of the caramelized onion mixture with the ground beef, breadcrumbs, Parmesan cheese, egg, milk, and seasonings. Mix to combine and then form into 1-inch balls.

Cook your meatballs until no longer pink inside, using your desired cooking method. We like to air fry the meatballs for 12 minutes at 375 degrees Fahrenheit.

Meanwhile, add 1 cup of beef stock to the remaining onions and simmer to turn into a sauce.

Once the meatballs are cooked, add them to an oven safe pan, top with the onion sauce, and Gruyere cheese.

Broil on low for a few minutes until cheese is melted, then remove from the oven and garnish with additional thyme.