1 1/2 tablespoons unsalted butter (3/4 ounce; 20g)
1 1/2 tablespoons (12g) all-purpose flour
1 cup (240ml) whole milk
Kosher salt and freshly ground black pepper
Freshly grated nutmeg, to taste (optional)
2 ounces (55g) finely grated Swiss or Gruyère cheese
In a small saucepan, melt butter over medium-high heat (do not allow it to brown). Add flour and whisk to form a paste. Continue to cook, stirring, until raw flour scent is gone, about 1 minute. Whisking constantly, add milk in a thin, steady stream, or in increments of a couple of tablespoons at a time, whisking thoroughly and getting into all corners of the pan to maintain a homogeneous texture. Sauce will initially become very thick, then get very thin once all the milk is added.
Heat, stirring, until sauce comes to a simmer and begins to thicken slightly. Reduce heat to low and cook, stirring, until sauce is thick enough to coat the back of a wooden spoon, about 3 minutes.
Season with salt and pepper. Add nutmeg to taste, if using. If any lumps form, simply whisk thoroughly to remove them and smooth out sauce, or use an immersion blender or countertop blender if lumps are particularly large or tough. Working in 2 or 3 additions, whisk in cheese, incorporating each addition just as the one before it is on the verge of fully melting; make sure to keep the heat low after the cheese goes in, as too much heat can cause the sauce to break. Remove from heat.
Use sauce right away, or press a piece of plastic wrap over surface of sauce to prevent a skin from forming and keep warm until ready to use. Sauce can be cooled and stored in a sealed container in the refrigerator for several days. Reheat very gently on the stovetop or in the microwave before use.