Cook the Pasta: Bring a large pot of salted water to a boil. Add the fettuccine and cook according to package directions until al dente, typically 8-10 minutes. The pasta should be tender but still have a slight bite to it. Drain well in a colander, shaking off excess water, and set aside. Do not rinse the pasta as the starch helps the sauce adhere better.
Make the Roux Base: In a large skillet over medium heat, melt the butter completely until it starts to bubble slightly. Add the all-purpose flour and whisk constantly for 30-60 seconds to create a smooth paste (roux). This cooking step is crucial to eliminate the raw flour taste while not allowing it to brown.
Create the Sauce Base: Gradually add the milk to the roux, whisking continuously to prevent lumps from forming. Pour in small amounts at first, creating a smooth mixture before adding more. Once all milk is incorporated, add the cream cheese pieces and whisk until completely melted and the sauce is smooth.
Season the Sauce: Add the freshly grated Parmesan cheese, chopped parsley, garlic powder, onion powder, salt, and pepper to the sauce. Stir consistently until all the cheese has melted and the sauce has thickened to a rich, creamy consistency that coats the back of a spoon. This should take about 3-5 minutes.
Incorporate the Crabmeat: Reduce the heat to low and gently fold in the lump crabmeat. Use a spatula or wooden spoon to carefully incorporate it without breaking up the delicate crab pieces. Allow the crab to warm through in the sauce for about 2 minutes, being careful not to overmix or cook the crab too long.
Combine and Serve: Add the cooked fettuccine to the sauce (or pour the sauce over individual portions of pasta). Toss gently to coat the pasta evenly with the creamy sauce and crabmeat. Serve immediately on warmed plates, garnished with additional chopped parsley and freshly grated Parmesan if desired.