Cook the spaghetti according to package instructions until al dente, then drain and set aside.
Season the flank steak with salt and black pepper on both sides.
Heat a large skillet over medium-high heat and add 2 tablespoons of butter.
Add the seasoned flank steak to the skillet and sear for 4-5 minutes on each side for medium-rare, then remove from the skillet and let rest.
In the same skillet, reduce heat to medium and add remaining butter and minced garlic, cooking until fragrant.
Pour in the beef broth and red pepper flakes, stirring to combine and reduce slightly.
Slice the rested steak against the grain into thin strips and add to the skillet with the sauce, followed by the cooked spaghetti.
Toss everything together gently, allowing the noodles to be coated in the sauce, then stir in the Parmesan cheese and chopped parsley.
Serve immediately, garnished with extra parsley and Parmesan if desired.
