Classic Mornay Sauce

Peel 1/2 small onion, carefully trimming around the root end so it is still intact. Place 1 dried bay leaf onto the flat part of the onion and use 3 whole cloves to pin the leaf to the onion, pressing the cloves firmly to secure it. (This is called an onion piqué, used to flavor the sauce.) Place in a small saucepan.

Add 2 1/4 cups whole milk and bring to a gentle simmer over medium heat. Reduce the heat to low and let the milk warm and infuse, stirring occasionally to prevent a skin from forming, for 10 minutes. Meanwhile, grate 2 ounces Gruyère cheese on the large holes of a box grater (about 1/2 cup). Finely grate 2 ounces Parmesan cheese on the smallest holes of the box grater (about 1 cup).

Remove the saucepan from the heat. Pour the milk mixture through a fine-mesh strainer into a liquid measuring cup. Reserve the onion piqué. Transfer 1/4 cup of the milk to a small bowl.

Give the saucepan a good rinse and wipe to remove any milk residue. Melt 3 tablespoons unsalted butter in the cleaned saucepan over medium heat, making sure the butter doesn't brown. Add 3 tablespoons all-purpose flour and cook, whisking continuously, for 2 minutes until foamy, bubbly, and pale yellow.

While whisking constantly, slowly pour in the the remaining 2 cups warmed milk about 1/4 cup at a time. It will initially aggressively bubble and instantly turn into a thick paste, then look like scrambled eggs, and finally become smooth and runny once all of the milk is whisked in.

Return the onion piqué to the saucepan. Reduce the heat to medium low and simmer, gently stirring with a wooden spoon and scraping against the botton and corners of the pan, until pourable but thick enough to coat the back of a spoon, about 5 minutes.

Remove and discard the onion piqué. Vigorously whisk the sauce again until smooth (or strain through a fine-mesh strainer if desired). Add 3/4 teaspoon kosher salt, 1/8 teaspoon ground white pepper, and 1/8 teaspoon freshly grated nutmeg if desired and stir to combine.

Reduce the heat to low. Add the cheeses a handful at a time, whisking after each addition. It will instantly seize up and thicken. To loosen, add the reserved milk a tablespoon at a time as needed until the desired consistency is reached. Add 2 teaspoons Dijon mustard if desired and whisk to combine. Taste and season with more kosher salt as needed. Give the sauce once final whisk until smooth. Serve immediately or press a sheet of plastic wrap directly onto the surface of the sauce to prevent a skin from forming to keep warm.