In a large mixing bowl, combine ground beef, egg, breadcrumbs, Parmesan, parsley, garlic, salt, and pepper. Mix gently until just combined and form 1-inch meatballs (20-24 total).

Heat a skillet over medium heat and add olive oil. Once hot, sear the meatballs until browned on all sides (6-8 minutes), then remove from skillet.

Reduce heat to low, add butter to the skillet, and melt. Stir in garlic and red pepper flakes, cooking until fragrant (1-2 minutes). Add lemon juice and remaining parsley. Return meatballs to skillet and simmer for 5-7 minutes.

Cook linguine according to package instructions in salted boiling water until al dente. Reserve 1 cup of pasta water, then drain. In a separate pot, melt more butter, add heavy cream, and bring to a gentle simmer. Stir in Parmesan and adjust consistency with pasta water.

Serve linguine topped with garlic butter meatballs, drizzled with remaining sauce, and garnish with parsley and Parmesan.