Preheat your oven to 400°F.
In a large bowl, combine ground beef, breadcrumbs, Parmesan cheese, chopped parsley, egg, minced garlic, salt, black pepper, and red pepper flakes (if using). Mix until just combined; do not overmix.
Shape the mixture into meatballs, about 1 inch in diameter, and place them on a baking sheet lined with parchment paper.
In a skillet over medium heat, melt the butter and olive oil together. Once hot, add the meatballs in batches, cooking for about 5-6 minutes, turning occasionally until browned on all sides.
Transfer the browned meatballs back to the baking sheet and bake in the preheated oven for 10-15 minutes, or until cooked through (internal temperature should reach 160°F).
Remove from the oven and garnish with additional chopped parsley before serving.