Preheat oven to 375°F (190°C) and line a baking sheet with parchment paper.
Dice the onion and sauté in a pan with butter until soft and translucent.
In a large bowl, combine ground beef, sausage, eggs, Parmesan cheese, and seasonings. Add the cooked onions to the bowl and mix everything together until just combined.
Wet your hands with water to prevent sticking, then form the mixture into meatballs.
Place meatballs on the baking sheet and bake for 20–25 minutes, or until they reach 160°F inside.
Serve with your favorite sauce and enjoy!