Melt the 4 tablespoons of unsalted butter in a heavy-bottomed saucepan over medium heat until it foams and bubbles.

Immediately whisk in the ¼ cup of all-purpose flour. Cook the flour and butter mixture, whisking continuously against the bottom of the pot, for 1 to 2 minutes. (This critical cooking step cooks out the raw flour taste and establishes a smooth, pale paste known as a roux, which acts as the thickening base for the sauce).

Ensure the whole milk is at room temperature to prevent shocking the hot roux and causing lumps.

Slowly pour the milk into the bubbling paste in a steady stream, whisking aggressively and continuously to break apart any clumps and unify the liquid.

Reduce the heat to medium-low. Continue cooking and whisking slowly until the sauce begins to visibly thicken and homogenize.

To test the viscosity, dip a stainless steel spoon into the sauce and run your finger through the coating on the back of the spoon. If the line holds its shape without the liquid weeping back together, the sauce has reached the proper thickness (about 5 minutes).

Remove the saucepan from the heat.

Vigorously whisk in the kosher salt, white pepper, ground nutmeg, and grated Parmesan cheese. The residual heat will melt the cheese into the sauce, yielding a perfectly glossy, rich mother sauce ready to be poured over vegetables, baked pastas, or layered into casseroles.