Prepare the beef properly by patting it dry with paper towels and seasoning generously with salt and pepper. This ensures good browning and flavor development.
Bring a large pot of salted water to boil for the fettuccine. The water should taste like the sea – about 1 tablespoon of salt per 4 quarts of water.
Heat 2 tablespoons olive oil in a large skillet over high heat until shimmering. Add the beef in a single layer (work in batches if needed) and sear for 1-2 minutes per side until browned but still slightly pink inside. Remove to a plate.
Reduce heat to medium-high and add remaining olive oil to the same skillet. Add mushrooms and cook undisturbed for 2 minutes, then stir and continue cooking for 3-4 minutes until golden brown and their moisture has evaporated.
Add 2 tablespoons butter, garlic, and shallot to the mushrooms and cook for 1 minute until fragrant, stirring constantly to prevent burning.
Pour in beef broth, soy sauce, and Worcestershire sauce, scraping the bottom of the pan to release any browned bits. Bring to a simmer and cook for 3-4 minutes until slightly reduced.
Meanwhile, cook the fettuccine according to package directions until al dente, usually 9-11 minutes. Reserve 1 cup of pasta water before draining.
Return the beef and any accumulated juices to the skillet with the mushroom sauce. Stir to combine and warm through, about 1 minute.
Add the drained pasta directly to the skillet along with the remaining 2 tablespoons butter and Parmesan cheese. Toss continuously using tongs until the butter melts and creates a silky sauce that coats the pasta. Add splashes of reserved pasta water as needed to reach desired consistency.
Taste and adjust seasoning with salt and pepper. Sprinkle with fresh parsley and additional Parmesan. Serve immediately with optional red pepper flakes for those who enjoy a kick.