In a large mixing bowl, combine ground beef, finely diced onion, breadcrumbs, egg, half of the minced garlic, salt, and pepper. Mix until just combined and shape into 16–18 meatballs.

Bring a large pot of salted water to a boil. Add linguine and cook according to package instructions until al dente, around 9–11 minutes. Reserve 1 cup of pasta water before draining.

In a large skillet, heat 4 tablespoons of butter over medium-high heat. Add the meatballs and brown for 8–10 minutes until golden brown. Remove and set aside.

In the same skillet, add remaining 4 tablespoons of butter and remaining garlic. Sauté for 1 minute, then add heavy cream and bring to a gentle simmer for 2–3 minutes.

Add the browned meatballs to the skillet and simmer for an additional 5 minutes. Then, add the cooked linguine and grated Parmesan, tossing gently to combine.

Serve hot, garnished with freshly chopped parsley and an extra sprinkle of Parmesan cheese.