Bring a large pot of generously salted water to a boil. Cook fettuccine until al dente according to package directions. Reserve 1/2 cup pasta water before draining.
While pasta cooks, melt butter in a large skillet over medium heat. Add minced garlic and cook for 1 minute until fragrant. Do not let it brown.
Whisk in flour and cook for 1 minute, stirring constantly. Slowly add heavy cream and milk, whisking continuously to prevent lumps. Cook for 3-4 minutes until sauce thickens and coats the back of a spoon.
Reduce heat to low. Gradually add grated Parmesan, one handful at a time, stirring until each addition is melted and smooth. Stir in nutmeg, salt, pepper, and red pepper flakes if using. Remove from heat.
Add drained pasta to the sauce and toss with tongs until every strand is coated. If sauce is too thick, add reserved pasta water one tablespoon at a time.
Serve immediately topped with fresh parsley, extra Parmesan, cracked pepper, and red pepper flakes.
