Soak breadcrumbs in milk for 5 minutes.

In a large bowl, combine ground beef, soaked breadcrumbs, parsley, thyme, herbes de Provence, salt, pepper, minced onion, garlic, ¼ cup Gruyere cheese, and whisked egg.

Mix the ingredients gently until combined, then form 2 tablespoon-sized meatballs and place them on a tray.

Chill the meatballs in the fridge for 15 minutes.

Heat a large cast iron skillet over medium-high heat and add 2 tablespoons olive oil.

Sear the meatballs until browned on all sides for about 8 to 10 minutes. Remove and set aside.

In the same skillet, add the remaining 2 tablespoons of olive oil and ghee or butter, then add the onions with a pinch of salt and pepper.

Cook the onions down for 20 to 25 minutes until caramelized.

Stir in garlic, herbes de Provence, and dried thyme, cooking until aromatic.

Add sherry and reduce for 30 seconds, then incorporate the flour and mix for 30 seconds.

Pour in beef stock, whisk until smooth, and let simmer for 5 minutes.

Stir in fresh thyme leaves, then return the meatballs to the skillet and top with Gruyere and mozzarella cheeses.

Broil for 5 to 7 minutes until the cheese is melted and gooey.

Serve the meatballs garnished with additional thyme leaves.