Start by bringing a large pot of salted water to a boil. Add the fettuccine pasta and cook according to package instructions until al dente—usually about 10-12 minutes. Once cooked, drain the pasta, reserving about ½ cup of pasta water for later. Set the pasta aside.
While the pasta cooks, heat 2 tablespoons of olive oil in a large skillet over medium-high heat. Pat the steak strips dry with paper towels and season generously with salt and pepper. Add the steak strips to the hot skillet in a single layer, working in batches if necessary to avoid overcrowding. Sear the steak for about 2-3 minutes per side until nicely browned but still tender inside. Remove the steak from the skillet and set aside.
Reduce the heat to medium, then add 4 tablespoons of butter to the same skillet. Once melted, add the minced garlic and sauté for about 1 minute until fragrant but not browned.
Slowly pour in 1 cup of heavy cream, stirring constantly. Allow the cream to warm and slightly thicken—this should take about 3-4 minutes. Then, gradually sprinkle in the grated Parmesan cheese, stirring continuously until the sauce becomes thick and creamy. If the sauce feels too thick, add a splash of the reserved pasta water to loosen it up to your desired consistency. Taste and season with salt and pepper as needed.
Add the cooked steak strips back into the skillet with the Alfredo sauce, stirring to coat the meat in the creamy garlic butter sauce. Next, add the drained fettuccine pasta to the skillet and toss everything together until the pasta is fully coated and heated through. If the sauce needs loosening, add more reserved pasta water, a tablespoon at a time.
Turn off the heat and sprinkle chopped parsley over the top for a burst of fresh green color and flavor. Serve immediately while warm and enjoy.