In a large bowl, combine ground beef, ground pork, beaten egg, cheese, parsley, salt, and pepper. Mix together until just combined.

In a medium bowl, combine the milk, breadcrumbs, garlic, and onion.

Add milk mixture to the meat mixture being careful not to overwork ingredients.

Form into desired size meatballs, greasing hands before starting to keep the meat from sticking, and place on a parchment-lined baking sheet.

In a large non-stick skillet over medium heat, melt butter and add oil. Once the oil is hot and shimmering, add the meatballs in batches, ensuring they don’t overcrowd the pan. Cook the meatballs, turning occasionally to brown all sides, until they reach an internal temperature of 165°F (8-10 minutes).

Remove the cooked meatballs and set them on a plate. Repeat with the remaining batches.

In a large oven-safe, non-stick skillet over medium-high heat, melt butter.

Once melted, add the onions and sauté for 25-30 minutes, stirring occasionally, until they are very tender and caramelized to a rich golden brown color.

Sprinkle flour over the onions. Stir to combine.

Add beef broth, salt, and pepper. Stir to combine. Cook for 7-10 more minutes, or until the sauce begins to thicken.

Nestle the cooked meatballs into the sauce in the skillet.

Evenly sprinkle mozzarella and Gruyere cheese over the meatballs.

Preheat the oven broiler.

Broil for 2-3 minutes, or until cheese is melting and bubbling.

Garnish with parsley. Serve warm.