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

In a medium bowl, combine whole milk, fine dry breadcrumbs, minced garlic, and finely diced yellow onion.

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

Form into desired size meatballs, greasing hands before starting, and place on a parchment-lined baking sheet.

In a large non-stick skillet over medium heat, melt unsalted butter and add vegetable oil. Once hot, add the meatballs in batches and cook until they reach an internal temperature of 165°F (8-10 minutes), turning occasionally to brown all sides.

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 2 tablespoons of unsalted butter.

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

Sprinkle all-purpose flour over the onions and stir to combine.

Add beef broth, kosher salt, and black pepper. Stir to combine and cook for 7-10 more minutes, 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 the cheese is melting and bubbling.

Garnish with chopped parsley and serve warm.