Melt the butter in a large skillet over medium-low heat. Add the sliced onion and stir to coat in the butter. Cook for 30-40 minutes until deeply browned and soft.

Preheat your oven to 400°F (200°C). In a mixing bowl, combine ground beef, half of the cheeses, breadcrumbs, parsley, grated onion, olive oil, garlic, salt, and pepper. Mix until just combined and shape into 12 meatballs.

Line a baking sheet with parchment and arrange the meatballs, leaving space between them. Bake for 15-18 minutes until golden brown and cooked through, reaching an internal temperature of 160°F (71°C).

Pour in beef stock with the caramelized onions and stir. Let it simmer over low heat for about 5 minutes to meld flavors.

Add the baked meatballs to the skillet with the onion mixture. Sprinkle remaining cheese over the meatballs and return to the oven for 15-20 minutes until cheese is melted and bubbly.

Garnish with parsley and serve warm with crusty bread.