In a large mixing bowl, combine ground beef, beaten egg, breadcrumbs, grated Parmesan, chopped parsley, minced garlic, salt, and black pepper. Mix until just combined and form into meatballs.

Heat olive oil in a large skillet over medium heat. Add meatballs in a single layer and sear for 6-8 minutes until golden brown on all sides. Remove and set aside.

In the same skillet, reduce heat to low and add unsalted butter and minced garlic. Sauté for 1-2 minutes until fragrant. Add red pepper flakes if desired.

Stir in lemon juice and chopped parsley into the sauce. Return the meatballs to the skillet and simmer on low for 5-7 minutes until cooked through.

Boil a large pot of salted water and cook linguine according to package instructions, about 8-10 minutes for al dente. Set aside pasta water before draining.

In a saucepan, melt butter and add heavy cream. Stir until gently simmering and add grated Parmesan until melted and smooth. Season with salt and pepper.

Toss the cooked linguine in the creamy sauce, adding reserved pasta water to reach desired consistency. Serve topped with meatballs and remaining sauce, garnished with parsley and Parmesan.