Boil the pasta: Bring a large pot of salted water to a rolling boil. Cook the pasta according to package directions until al dente. Reserve 1 cup of pasta water, then drain.

Warm the milk: In a small pot or microwave-safe cup, gently warm the milk until just steamy. This helps the sauce come together smoothly.

Make the roux: In a large skillet or wide pot, melt the butter over medium heat. Add the flour and whisk for 1–2 minutes until it smells toasty but not browned. Keep it moving so it doesn’t clump.

Add garlic: Stir in the minced garlic and cook 30 seconds. Avoid browning the garlic, which can add bitterness.

Whisk in milk gradually: Slowly pour in the warm milk while whisking constantly. Start with a few splashes to form a smooth paste, then add the rest in a steady stream. Cook 4–6 minutes, whisking, until the sauce thickens and coats the back of a spoon.

Season the sauce: Add salt, black pepper, a tiny pinch of nutmeg, and red pepper flakes if using. Stir in the heavy cream if you want a richer sauce.

Add Parmesan: Turn heat to low and stir in the Parmesan until melted and smooth. If it gets too thick, loosen with a splash of the reserved pasta water.

Sauté add-ins (optional): In a separate pan, warm the olive oil over medium heat and sauté mushrooms until golden, or quickly heat pre-cooked chicken or shrimp. Lightly season with salt and pepper.

Combine pasta and sauce: Add the drained pasta to the sauce and toss until every piece is coated. Fold in your add-ins. Adjust seasoning and consistency with more pasta water as needed.

Finish and serve: Sprinkle with parsley, add a little extra Parmesan on top, and serve immediately while hot and glossy.