Cook fettuccine al dente in salted water according to package directions. Reserve 1 cup pasta water, drain and toss the pasta with a drizzle of olive oil if not immediately adding the pasta to the sauce (prevents it from clumping).
Melt the butter in a large saucepan over medium heat. Meanwhile, whisk the milk with the cornstarch until completely smooth. Add the garlic to the melted butter and sauté for 30 seconds.
Whisk in the heavy cream, milk/cornstarch, cream cheese and all seasonings. Reduce heat to medium-low and cook until melted and smooth.
Add the grated Parmesan cheese a handful at time in 3-4 batches, whisking until smooth in between each addition.
Bring the sauce to a gentle simmer and continue to cook for about 3-5 minutes or until it starts to thicken and the Parmesan is completely melted. Keep in mind it will continue to thicken once it cools slightly and when you add the pasta.
Add the fettuccine and toss to combine (if you're not serving immediately, wait to add the pasta). If at any time the Alfredo sauce gets too thick, just add a little splash of the reserved pasta water to loosen it up.
Taste and season with additional freshly cracked salt and pepper to taste (we like both). Garnish with fresh parsley if desired. Serve immediately.