Bring a large pot of salted water to a boil and cook the fettuccine until al dente according to package directions; reserve 1 cup pasta cooking water, then drain.
In a large saucepan over medium heat, melt the butter. Add the minced garlic and sauté about 30 seconds until fragrant.
While the butter melts, whisk together the milk and cornstarch until smooth and lump-free.
Add the heavy cream, the milk–cornstarch mixture, softened cream cheese (if using), dried basil, dried parsley, onion powder, dried oregano, salt, pepper, red pepper flakes, and nutmeg to the pan; reduce heat to medium-low and cook, stirring, until the cream cheese melts and the mixture is smooth.
Gradually add the grated Parmesan in 3–4 handfuls, whisking until smooth after each addition so the cheese fully melts into the sauce.
Bring the sauce to a gentle simmer and cook 3–5 minutes, stirring, until it thickens slightly (it will continue to thicken as it cools).
Add the drained fettuccine to the sauce and toss to coat; if the sauce is too thick, loosen with splashes of the reserved pasta water until desired consistency is reached.
Taste and adjust seasoning with additional salt and pepper if needed, then serve immediately and garnish with parsley if desired.
