Melt the butter in a medium-sized skillet over medium-high heat, then gradually whisk in the flour until a smooth mixture forms.
Gradually incorporate the milk and heavy cream into the flour mixture, whisking continuously until smooth. Bring the sauce to a gentle simmer, stirring occasionally until it thickens slightly.
Take the skillet off the heat and whisk in the Parmesan cheese and pesto until fully combined.
At the same time, cook the fettuccine according to package instructions, ensuring to reserve 1 cup of the pasta water before draining.
Toss the cooked pasta with the sauce, ensuring the noodles are well coated.
Gradually add the reserved pasta water until the sauce reaches a creamy consistency.
Season with salt and pepper to your liking and serve the pasta warm, with additional Parmesan cheese if desired.