Grate the Parmesan (if not pre-grated) and measure the milk and cream. Mince the garlic and chop parsley. Tear or slice mozzarella and set aside at room temperature so it melts easily.

In a medium heavy-bottomed saucepan over medium heat, melt the butter until foamy but not browned, about 1–2 minutes.

Add the minced garlic to the melted butter and cook, stirring, for 30–45 seconds until fragrant. Do not let it brown.

Sprinkle the flour over the butter and garlic and stir to combine, cooking the roux for 1 minute to remove raw flour taste. It should be bubbling gently but not darkening.

Slowly whisk in the whole milk until smooth, then whisk in the heavy cream. Bring the mixture to a gentle simmer over medium-low heat, whisking occasionally, for 3–4 minutes until slightly thickened.

Reduce heat to low. Gradually add the grated Parmesan in small handfuls, whisking constantly until the cheese fully melts and the sauce is smooth. If the sauce becomes too thick, whisk in up to 2 tablespoons additional milk to reach desired consistency.

Season the sauce with salt, freshly ground black pepper, and the optional grated nutmeg. Taste and adjust seasoning. Keep the sauce warm on the lowest heat, stirring occasionally so it doesn’t cling to the bottom.

Assembly - Mozzarella & Finish

Preheat oven broiler (or oven to 425°F / 220°C if you prefer baking). If using broiler, arrange an oven rack about 6 inches from the heating element.

Transfer the warm Alfredo sauce to an oven-safe shallow baking dish (or individual ramekins) in an even layer. If serving over pasta, toss pasta with sauce in a baking/serving dish instead.

Evenly distribute the sliced or torn fresh mozzarella over the top of the warm sauce so pieces are slightly overlapping.

Place the dish under the broiler for 1–3 minutes, watching closely, until the mozzarella is melted and just beginning to bubble and lightly brown. If baking at 425°F, bake 6–8 minutes until melted and bubbly.

Remove from oven. Drizzle with the optional extra-virgin olive oil and sprinkle with chopped fresh parsley and optional red pepper flakes for a touch of heat.

Serve immediately: spoon over cooked pasta, grilled chicken, or crusty bread. Store leftovers refrigerated in an airtight container for up to 3 days; reheat gently on the stove with a splash of milk or cream to loosen sauce.