Bring a large pot of salted water to a boil and cook the fettuccine until al dente according to package directions. Reserve 1 cup of the pasta cooking water, then drain the pasta and toss with a drizzle of olive oil if not adding to the sauce immediately.
In a large saucepan over medium heat, melt the butter. Add the minced garlic and sauté about 30 seconds until fragrant.
In a small bowl or measuring cup, whisk the milk together with the cornstarch until completely smooth and lump-free.
Whisk the heavy cream, the milk/cornstarch mixture, the soft cream cheese (if using), basil, parsley, onion powder, oregano, salt, pepper, red pepper flakes, and nutmeg into the melted butter and garlic. Reduce heat to medium-low and cook, stirring, until smooth and combined.
Add the grated Parmesan in 3–4 handfuls, whisking until each addition is fully melted and the sauce is smooth before adding more.
Bring the sauce to a gentle simmer and cook 3–5 minutes until it begins to thicken; it will thicken more as it cools and when combined with the pasta.
Add the drained fettuccine to the sauce and toss to coat. If the sauce is too thick, loosen with a splash of the reserved pasta water until you reach the desired consistency.
Taste and adjust seasoning with additional salt and freshly cracked pepper if needed, then serve immediately, garnishing with parsley if desired.