Cook the pasta. Boil linguine in salted water until al dente. Reserve 1 cup of starchy pasta water before draining.
Brown the beef. In a large skillet over medium-high heat, cook the ground beef and diced onion until the beef is browned and onions are soft (6-8 minutes). Drain excess grease.
Make garlic butter. Lower heat to medium-low. Push beef to the edges, melt butter in the center, and add minced garlic, Italian seasoning, and red pepper flakes. Cook for 1 minute until fragrant.
Add the cream. Stir the garlic butter into the beef. Pour in the heavy cream and let it simmer and thicken slightly for 2 minutes.
Combine and toss. Reduce heat to low. Add the drained pasta, parmesan, salt, and pepper to the skillet. Pour in 0.5 cup of reserved pasta water and toss vigorously until a smooth sauce forms. Add more water if needed. Garnish with parsley and serve.
