Bring a large pot of salted water to a boil. Cook the pasta until al dente, about 8–10 minutes. Reserve 1/2 cup pasta water, then drain and set aside.
Pat the steak dry and season both sides with salt and black pepper. Heat olive oil in a skillet over medium-high heat. Sear the steak for 3–4 minutes per side, or until desired doneness. Remove and let rest for 5–10 minutes.
In the same pan, melt butter and add the minced garlic. Sauté for 1–2 minutes until fragrant and golden, then stir in red pepper flakes.
Add the cooked pasta to the pan and toss in the garlic butter sauce, using reserved pasta water to loosen the sauce if needed.
Slice the rested steak thinly against the grain.
Plate the pasta and top with sliced steak. Garnish with chopped parsley and Parmesan if desired. Serve immediately while hot.
Notes
For extra flavor, let the steak marinate for a few hours before cooking.