Bring a large pot of salted water to a boil. Add fettuccine pasta and cook according to package instructions until al dente. Drain and set aside.
Pat the sirloin steak cubes dry with paper towels. Season with kosher salt and black pepper evenly.
Heat olive oil in a large skillet over medium-high heat. Add steak bites in a single layer and cook without stirring for 2-3 minutes to develop a crust.
Stir the steak bites and continue cooking for another 2-3 minutes until browned and cooked to desired doneness. Remove steak bites from skillet and set aside, keeping warm.
In the same skillet, reduce heat to medium and add unsalted butter. Once melted, add minced garlic and sauté for 1 minute until fragrant, stirring frequently to avoid burning.
Pour in heavy cream, garlic powder, and crushed red pepper flakes. Stir and cook for 2-3 minutes until the sauce slightly thickens.
Add the cooked pasta to the skillet and toss to coat evenly with the sauce.
Remove the skillet from heat and stir in freshly grated Parmesan cheese until melted and creamy.
Return the steak bites to the skillet and gently toss with the pasta and sauce to combine.
Sprinkle chopped fresh parsley over the top and serve immediately.