Bring a large pot of salted water to a boil. Add the 8 oz fettuccine pasta and cook according to package instructions until al dente, about 10-12 minutes. Drain the pasta, reserving ½ cup of pasta water, and set aside.
While the pasta cooks, heat a large skillet over medium-high heat. Add half of the butter (2 tbsp). Once melted, add the steak strips. Season with salt and black pepper. Sear the steak for 2-3 minutes per side, or until browned but still tender inside. Remove the steak from the skillet and set aside.
In the same skillet, add the remaining butter. Toss in the sliced mushrooms and cook until they release their moisture and become golden brown, about 5 minutes. Add the minced garlic and red pepper flakes (if using) and sauté for another 1-2 minutes until fragrant.
Pour in the beef broth to deglaze the pan, scraping up any browned bits from the bottom. Let the broth simmer for 2-3 minutes to reduce slightly and concentrate flavor.
Return the cooked steak strips to the skillet. Add the drained fettuccine and toss everything together, coating the pasta in the garlic butter sauce. If the sauce seems too thick, add a splash of the reserved pasta water to loosen it up.
Sprinkle the chopped fresh parsley over the pasta for a burst of color and freshness. Serve immediately while warm and enjoy!
