Bring a large pot of salted water to a boil. Add 8 oz of fettuccine pasta and cook according to package instructions until al dente, usually about 10-12 minutes. Once cooked, drain the pasta and set it aside, reserving about ½ cup of pasta water for later.
Heat 2 tablespoons of olive oil in a large skillet over medium-high heat. Add the thinly sliced beef sirloin in a single layer, making sure not to overcrowd the pan. Season with salt, pepper, and 1 teaspoon garlic powder. Sear each side for 2-3 minutes until browned but still tender. Remove the beef from the pan and set aside.
In the same skillet, add the sliced mushrooms. Cook them for 4-5 minutes until they release their moisture and develop a golden-brown color. Stir occasionally to prevent burning.
Pour in 1 cup of beef broth, scraping the bottom of the pan to lift any browned bits. Add 1 tablespoon of balsamic vinegar and stir well. Let the sauce simmer for 3-4 minutes, allowing it to reduce slightly and intensify in flavor.
Return the seared beef back to the skillet, mixing it with the mushrooms and sauce. Add the cooked fettuccine and toss everything together. If the sauce feels too thick, add a splash of the reserved pasta water to loosen it up to your desired consistency.
Sprinkle 2 tablespoons of fresh chopped parsley and a generous amount of grated Parmesan cheese over the pasta. Serve immediately and enjoy the delightful combination of flavors.