Preheat the oven to 400°F (200°C) and line a baking sheet with parchment paper.
In a large mixing bowl, combine the ground beef, ground pork, breadcrumbs, Parmesan cheese, Italian seasoning, salt, black pepper, minced garlic, red pepper flakes (if using), and parsley. Mix until just combined, being careful not to overwork the meat.
Form the mixture into meatballs, about 1 to 1.5 inches in diameter, and place them on the prepared baking sheet. Ensure they are spaced evenly apart.
Bake the meatballs in the preheated oven for about 20-25 minutes, or until they are cooked through and browned on the outside.
While the meatballs are baking, prepare the garlic butter sauce. In a skillet over medium heat, melt the unsalted butter. Add the minced garlic and cook for 1-2 minutes until fragrant, being careful not to let it burn.
Pour in the chicken broth and lemon juice, stirring to combine. Let the sauce simmer for 3-4 minutes to deepen the flavors.
Once the meatballs are done baking, transfer them to the skillet with the garlic butter sauce. Toss gently to coat the meatballs in the sauce, allowing them to simmer in the sauce for an additional 2-3 minutes.
Remove from heat and serve the garlic butter meatballs warm, garnished with additional parsley if desired.