Season the steak strips generously with salt and pepper. Heat olive oil in a large skillet or Dutch oven over medium-high heat. Add the steak in a single layer and sear for 1-2 minutes per side until browned. Do not overcrowd the pan; work in batches if necessary. Remove the steak and set it aside.

Reduce the heat to medium. Add the butter to the skillet. Once melted, add the diced onion and cook until softened, about 3-4 minutes. Add the minced garlic and red pepper flakes and cook for another minute until fragrant.

Pour in the beef broth, scraping up any browned bits from the bottom of the pan. Stir in the crushed tomatoes, heavy cream, and Italian seasoning. Bring the mixture to a simmer.

Add the uncooked pasta to the skillet. Stir well to combine, ensuring the pasta is mostly submerged in the liquid. Let it simmer, stirring occasionally, for 12-15 minutes, or until the pasta is al dente and the sauce has thickened.

Stir in the grated Parmesan cheese until it has melted into the sauce. Return the seared steak to the skillet and stir everything together to heat through. Garnish with fresh parsley and serve immediately.