Cook pasta according to package directions, reserving 1/2 cup pasta water.
In a skillet, heat olive oil and brown beef for 2 minutes. Season with salt and pepper, then add garlic, onion, chili flakes, Worcestershire sauce, tomato paste, parsley, and pasta water. Simmer for 8-10 minutes.
Toss in pasta and 1 cup mozzarella. Top with remaining cheese and cover.
Garnish with additional parsley before serving.
Notes
For best results, do not rinse the pasta after cooking.