Bring a large pot of salted water to a boil. Cook pasta until al dente according to package directions.
Meanwhile, pat steak dry, season with salt and pepper, and sear in 1 tablespoon olive oil for 1-2 minutes per side. Remove steak to a plate and cover.
In the same pan, add remaining oil and butter. Sauté onions until translucent (3-4 minutes), then add garlic and cook for 30 seconds until fragrant.
Pour in beef broth, scraping up browned bits from the pan. Reduce by half, then add heavy cream and Italian seasoning. Simmer 3-4 minutes until slightly thickened.
Stir in Parmesan cheese until melted.
Slice the rested steak against the grain into thin strips.
Drain pasta, reserving ½ cup pasta water. Add pasta to the sauce, tossing to coat. Add reserved pasta water if needed to reach desired consistency.
Fold in sliced steak and any accumulated juices. Heat through for 1 minute, then taste and adjust seasoning.
Serve garnished with fresh parsley and additional Parmesan cheese.